Final answer:
The sentence that uses a comma correctly is: 'With much difficulty, Grant finally completed the birdhouse he was building.'
Step-by-step explanation:
The sentence that uses a comma correctly is: With much difficulty, Grant finally completed the birdhouse he was building. The comma is used correctly in this sentence to separate the introductory phrase 'With much difficulty' from the main clause 'Grant finally completed the birdhouse he was building.' This use of the comma helps to clarify the meaning of the sentence.
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