Correct Answer: A
Explanation:
The key phrase that indicates how the blank for this question should be completed is
“the ability to form an unending series of different molecules.” Among the answer
choices, “diversity” and “variety” clearly fit logically with “unending” and “different”
and create two very similar sentences. No other pair of choices here would produce
two sentences as similar in meaning as those created by placing “diversity” and “variety”
in the blank. Thus the correct answer is diversity (Choice A) and variety (Choice C).