Let's take an example. An adverb is basically an adjective for a verb: it describes a verb, and often ends in -ly. Here's a sentence, WITHOUT PROPER PUNCTUATION: "Slowly Anna walked." What fits here?
Let's work backwards. If D is correct: "Slowly. Anna walked." This is incorrect, because this would make "slowly" part of a separate sentence, not the beginning of the same sentence. This is the same situation for C. If it were correct: "Slowly! Anna walked." This makes it a separate sentence, so C is incorrect. What about B? "Slowly; Anna walked." This is incorrect because it makes "slowly" too separate from "Anna walked." Semicolons are used for completely separate ideas, NOT adverbs. The only right answer is A: "Slowly, Anna walked." This separates the adverb enough so it doesn't confuse, but you still know we are really saying "Anna walked slowly."
At this extreme altitude gas molecules are widely separated. Above 60 miles (100 km) from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases (atomic oxygen, helium and hydrogen).
The frown on your face tells me that the test was harder than you expected. They often ask my friends and me if we will help them with their yard work.
