单选题
(Even though) Sedat (has been studying) English for three years before he came (to the United States), It is still difficult for him (to express himself).
【正确答案】
B
【答案解析】把has been studying改成had been studying(过去完成时)，因为 study English这个动作发生在he came to the United States之前，came to是过去时，所以study English是“过去的过去”，应该用过去完成时，而不是现在完成时。