Answer:
D When World War ll broke out
Explanation:
A clause is a group of words with a subject and a verb that add more to the sentence. The first one has a verb but no subject. The second one has a subject but no verb. So the last two could be confusing because they have both, but C doesn't say who served in the U.S Army so it could be confusing to the reader, so therefore the answer is D.
