Obsidian is a kind of volcanic glass formed when lava cools so quickly that the atoms inside it cannot arrange themselves in
a crystalline structure. You ______ more about obsidian’s structure, which is classiﬁed as amorphous, in a later chapter.
had learned
had been learning
will learn
have learned
Choice C is the best answer. The convention being tested is the use of verbs to express tense in a sentence. In this choice,
the future tense verb “will learn,” used in conjunction with the phrase “in a later chapter,” correctly indicates that “you” (the
reader) are going to learn about obsidian’s structure at some point in the future.
Choice A is incorrect because the past perfect verb “had learned” doesn’t indicate that the subject is going to learn about
obsidian’s structure in the future. Choice B is incorrect because the past perfect progressive verb “had been learning” doesn’t
indicate that the subject is going to learn about obsidian’s structure in the future. Choice D is incorrect because the present
perfect verb “have learned” doesn’t indicate that the subject is going to learn about obsidian’s structure in the future.