The part of this sentence following the semicolon tells us what is vital: the timing at which the vault's keys are turned. Since we are given no further information, we can assume that our answer will refer to multiple actions that happen at the same instant. "Simultaneous" is our answer, since it designates actions that occur concurrently (at the same time).

We can infer from the sentence that we need to pick out an adjective that can describe organisms that have “all” been “killed off.” While both “extinct” and “extant” might look like potentially correct answers because they look and sound similar, “extant” is an adjective that is used especially to describe a document and means still in existence. This is clearly an antonym of the word we’re looking for, so “extant” cannot be the correct answer. “Extinct,” however, is an adjective that can mean no longer in existence; so, because “extinct” best describes the dinosaurs after they were all killed off by something, “extinct” is the correct answer.

We know from the sentence that "snowstorms are extremely rare" in the tropics, so we need to pick out an adverb that means something like "rarely" to describe the interval at which they occur in the tropics. So, neither "always" and "usually" can be the correct answer, because each of those words is an antonym of "rarely." "Seldom," however, is an adverb that means rarely, and because it best fits the context of the sentence, "seldom" is the correct answer.

Because the sentence is comparative and begins, "Though all of the flowers smelled wonderful," we can infer that we need to pick out a word that means smelling good. While "fragrant," "rank," and "fetid" all describe smells, "rank" and "fetid" describe disgusting smells, so they wouldn't make sense in the sentence, comparing hyacinths to other "flowers [that] smelled wonderful." "Fragrant," however, is an adjective that means having a nice smell, and because "fragrant" best describes the smell of a flower, "fragrant" is the correct answer.

Students should be "attentive," or paying attention, if they want to absorb the material taught during class. "Careful" means cautious; "outgoing" means friendly and extroverted; and "distracted" means not fully paying attention to something.

In this sentence, the blank should include a word that describes the opposite of staying in the United States. The word that would make the most sense in the blank is "abroad," which means in a foreign country. None of the other answer choices would make sense in the blank: "campaign" means run for a political position, like for president; "intercept" means interrupt; "perish" means die; and, "plummet" means sharply decrease.

We need to pick out an answer choice that means something like "surprised" or "confused" because the sentence tells us that the audience thought that Larry was sure to win the Presidential Award. So, "shocked," an adjective that means surprised, is the correct answer choice because it best fits the sentence's context. None of the other answer choices would make sense in the blank space: "annoyed" means irritated; "tired" means exhausted; and "ecstatic" means very excited or elated.

While all the answers fit in the sentence grammatically, "exhausting" most closely relates to the meaning and context of the sentence.

The best word to describe fossils that are secured in limestone and sealed over time is embedded.

The sentence is saying that since the forest is so quiet, even soft sounds can be distinctly heard. Another way of saying this is to describe the sounds as "perceptible," or easily perceived.