Mashaal Masha
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.

Which choice completes the text so that it conforms to the conventions of Standard English?
Difficulty: Easy
A: 

had learned

B: 

had been learning

C: 

will learn

D: 

have learned

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