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What is the most effective way to combine these sentences?
A. After a two-and-a-half-hour trip and after he reached his target altitude, he then ran through an extensive checklist.
B After a two-and-a-half-hour trip, he reached his target altitude, then he ran through an extensive checklist.
C After a two-and-a-half-hour trip, he reached his target altitude, where he ran through an extensive checklist.
D After a two-and-a-half-hour trip where he ran through an extensive checklist, he reached his target altitude.
What is "c"