Explanation
Correct answer: A.
Choice A is the best answer. “Hence” logically signals that the information in this sentence—that geologists couldn’t confirm the theory of cyclical Ice Ages until the 1960s—is a consequence of the previous information about the destructiveness of each Ice Age and the erasure of necessary geological evidence.
Why the other choices are wrong
Choice B
Choice B is incorrect because “moreover” illogically signals that the information in this sentence is merely additional to the previous information about the destructiveness of each Ice Age. Instead, the sentence identifies a specific consequence of that information.
Choice C
Choice C is incorrect because “nevertheless” illogically signals that the information in this sentence is true despite the previous information about the destructiveness of each Ice Age. Instead, the sentence identifies a specific consequence of that information.
Choice D
Choice D is incorrect because “next” illogically signals that the information in this sentence is the next step in a process. Instead, the sentence identifies a specific consequence of the previous information.