Take another look at the item:

Smiling with satisfaction (A) Michelle typed the last sentence of her research paper. Then a bolt of lightning crashed nearby, (B) extinguishing the electricity (C) and making poor Michelle wish that she had invested in a surge protector.

You wanted to do this:

Smiling with satisfaction Michelle typed the last sentence of her research paper. Then a bolt of lightning crashed nearby. Extinguishing the electricity and making poor Michelle wish that she had invested in a surge protector.

A period after nearby makes a fragment. Extinguishing the electricity ... is a participle phrase, so it cannot stand alone as a complete sentence.

You might want to review the rules.

Go back to the sentence to try again.

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