单选题 Ms. Silva sent the memo ______ it had been approved.

A、 (A) so
B、 (B) but
C、 (C) after
D、 (D) until

【正确答案】 C

【答案解析】After is the correct subordinate conjunction showing a logical time relationship. Choice (A) shows cause and effect. Choice (B) expresses opposition. Choice (D) means till that time and no longer.