Which fragment could you correct by adding only a complete subject? revealed much about their behavior insect studies by closely watching and gathering data to terrifying effect

"revealed much about their behavior."

A fragment is a sentence that does not represent an independent clause and cannot stand on its own. An independent clause requires a subject and a verb, while a fragment does not have one of this requirements. In this case, the fragment that could become an independent clause through the addition of a complete subject is the first one. The sentence would look something like this:

Ex. [The old photographs] revealed much about their behavior.

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