Take a look at 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 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 electricity makes a fragment. And making poor Michelle wish ... 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.