C. At that time, modifies had.
The adverb phrase in the sentence is C) "at that time", and it modifies the verb "had".
Adverb phrases typically modify verbs, adjectives, or other adverbs, providing additional detail about how, when, where, or to what extent something occurred. In this case, "at that time" tells us when people had very few books.
Let's break down the options:
A. "People had" does not form an adverb phrase.
B. "very few books" is a noun phrase and modifies "books".
C. "at that time" is the correct adverb phrase, modifying the verb "had".
D. "very few" is part of the noun phrase "very few books", not an adverb phrase.
In summary, understanding adverb phrases and their modifying roles is essential for parsing sentences accurately.
The adverb phrase in the sentence is "at that time," and it modifies the verb "had." This provides information on when the event of having very few books occurred. Therefore, the correct answer is C.
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