21.
Your Answer is
Correct Answer is C
Explanation
This question investigates the omission of adverbial clauses. When the subject of the main clause in the adverbial clause is equal to the subject of the clause, and the clause contains a be verb, the subject + be verb in the clause can be omitted. According to the sentence meaning, while still in the Marines omits the subject he and the verb be, so the subject of the main clause is also he. Exclude A/B/D.