Here is the item again:

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 correctly realized that a comma was necessary after satisfaction. Smiling with satisfaction is a participle phrase. When a phrase of this type introduces a main clause, you need a comma to connect the two parts.

To fix this problem, you chose 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.

You have correctly employed Punctuation Rule 4:

Participle Phrase + , + Main Clause.

Well done!

Go to the next sentence.

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