The key word to consider when answering this question is the conditional word "could" in the last portion of the sentence. Because the forecasted shop drop in oil prices is discussed in conditional terms after the comma, it must also be discussed in conditional terms before the comma, and in the way the sentence is presented, it is not. The only answer choice that features appropriate wording is "Forecasters warn that a sharp drop in oil prices"; the "that" allows for the conditional, in the sense of the forecasters warning that if a sharp drop in oil prices occurs, it could have dire consequences for the world economy.

Eliminate the answer choices which include the passive voice. This question also has to do with order of relevance. Consider that the initial prepositional phrase describes the guide's most relevant action. What is the most important thing the guide did? He cleared away the debris. That should be mentioned in the active voice before the fact that he allowed his group to follow his path. Choose the answer that most efficiently describes the primary action (in the active voice) and sets up the clauses about the consequences of his action.

Eliminate the answer choices which include the passive voice. This question also has to do with order of relevance. Consider that the initial prepositional phrase describes the carpenter’s most relevant action. What is the most important thing the carpenter did? He split the log down the middle. That should be mentioned in the active voice before the fact that his action allowed the wood to sit in the fireplace. Choose the answer that most efficiently describes the primary action (in the active voice) and sets up the clauses about the consequences of his action.

The "as" at the beginning of the sentence’s final clause, "as the study of…" makes that clause dependent—it can’t stand on its own. Thus, the underlined clause must remain independent, or else the sentence will become a fragment. 

The word "seismologist" must be linked to noun phrase that begins "no specialized term…" with a word that makes sense in context and is idiomatically correct.

"Other than" makes perfect sense in context, and adding a conjunction to the beginning of the sentence would leave us with two dependent clauses.

Eliminate the answer choices that include the passive voice. This question also has to do with order of relevance. Consider that the initial prepositional phrase describes the forester's most relevant action. What is the most important thing the forester did? He cleared away the debris. That should be mentioned in the active voice before the fact that his action allowed his vehicle to pass. Choose the answer that most efficiently describes the primary action (in the active voice) and sets up the clauses about the consequence of his action. If the subject of the first clause is the forester "swinging" his axe, he must be the subject of the second clause to avoid a dangling modifier error.

One can distinguish a from b, or distinguish between a and b. The correct answer is the only one that follows this idiomatic pattern.

Present tense is always preferable when referring to existing works. The book was written in the past but still exists in the present. 

"February" is presumed to be in the future which requires the future tense "will." Adding "will" to "have been married" makes the future perfect tense. "Will be" is incorrect because it is the Simple Future tense which means that we will be married in February and then 10 years thereafter. 

There is no punctuation between the verb and its object. Additionally, the comma goes inside the quotation marks. Any combination of punctuation makes the other answers incorrect. 

The past progressive tense of "to lie" is "were lying." The auxiliary verb "were" completes the sentence. The difference between lay and lie revolves around the rule that to "lie" means "to rest" whereas "lay"means "to put or place." In this example, the debris and trees were placed on the ground by the hurricane.