There are no errors in this sentence. The phrase "In the past century" is an example of an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence.

Note that the choice with the phrasing "were not changed" is incorrect because it is in the passive voice, which is an unnecessary, if not strictly grammatically incorrect, change to make. Additionally, you do not need to rephrase the sentence to say that people changed engines, as the phrasing of this sentence is already correct and is - in fact - more concise. The correct answer choice is, "In the past century, engines have not changed."

This sentence does not contain any errors. The phrase "who would otherwise not have the opportunity" is an example of a relative clause, which gives more information about its antecedent: people. Any such phrase that occurs in the middle of a sentence must be introduced and completed with a comma. The correct choice is, "the Internet in the world is allowing many people, who would otherwise not have the opportunity, to share their experiences. "

This sentence contains no errors. This is a complex sentence with various clauses. The phrase "In addition to autonomous vehicles" is an example of an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence.

"many companies are still working on alternative modes of transportation" is the main clause of this sentence. The following part, "such as flying cars, steam- or air-powered cars, and changes to current public transportation" is a parenthetical expression: a phrase that explains more about a particular word or phrase in the sentence, but is not introduced by a relative pronoun. These phrases must be introduced and completed with a comma in order to separate them from the main clause of the sentence. Furthermore, the items in this expression are in a list, so they should be separated by commas.

Last, the word "steam-" should have the hyphen after it in order to indicate that it is part of the compound word "steam-powered." It is shorter alternative to saying "steam-powered and air-powered." Without the hyphen, the word "steam" should be treated as separate and unrelated to the word "air-powered," which then makes the phrase confusing.

The correct choice is, "many companies are still working on alternative modes of transportation, such as flying cars, steam- or air-powered cars, and changes to current public transportation."

This sentence contains no errors. The sentence combines two independent clauses with the use of the coordinating conjunction "but" and a comma. This is the correct usage. No other punctuation or changes are necessary. The correct choice is, "his inventions, but he was never satisfied with the end results."

Note that a single comma should not interrupt a clause, and that coordinating conjunctions cannot follow semicolons being used to join two independent clauses.

There are no errors in this sentence. The phrase "was fascinated" is not in the passive voice because it does not mean that someone else is fascinating Marla in this sentence. The verb here is actually "was," simply acting as a linking verb with "fascinated" describing Marla. Changing the "was" to "is" or omitting it actually does not make any sense grammatically. Additionally, the sentence combines two independent clauses with the coordinate conjunction "so," so a comma is necessary before the conjunction. The correct answer replicates the underlined portion of the example sentence.

This sentence contains, while somewhat awkward, contains no grammatical errors. The phrase "As Kelsey was writing the last parts of her book" is an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence. The main clause, "her team worked on: formatting, marketing, and everything else that would make her book successful" contains a list of parallel phrases which must be separated with commas. No corrections are necessary. The correct choice is, "As Kelsey was writing the last parts of her book, her team worked on formatting, marketing, and everything else that would make her book successful."

This sentence contains no errors. The phrase "which was supposed to arrive in ten minutes" is an example of a relative clause, which gives more information about its antecedent: the airplane. Any such phrase that occurs in the middle of a sentence must be introduced and completed with a comma. The correct answer choice is, "that the airplane, which was supposed to arrive in ten minutes, would not arrive."

This sentence contains no errors. The phrase "Every time the company would announce a new product" is an example of an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence. The rest of the sentence does not need any sort of punctuation because it is a simple phrase. The correct choice: "would announce a new product, people would line up outside the stores for days in anticipation of its release."

This sentence contains no errors. The sentence combines an independent clause to a dependent clause with the use of a colon. This is correct because colons are used to combine clauses when the second one explains something about the first clause, provided that the first clause is an independent clause (note that a semicolon can only be used to join two independent clauses). Furthermore, the word "Smooth Criminal" must be in quotes because it is the name of a song (same rule as poems, short stories, etc.). The correct choice is, "while she worked: Michael Jackson's "Smooth Criminal." "

This sentence contains no errors: it is a simple sentence. A colon is unnecessary because it comes after a dependent clause in the available choices (not possible with colons). The commas are also unnecessary because it is inappropriate to use a comma to introduce a prepositional phrase, as in "in life," or to separate an adverbial phrase: "where she grew up." The correct choice is, "Her dream in life was to become the mayor of the town where she grew up."