555.They ______ on the program for almost one week before I joined them,and now we ______ on it as no good results have come out so far.
A.have worked;are still working
B.had worked;were still working
C.have been working;have worked
D.had been working;are still working
556.The young girl sitting next to me on the plane was very nervous.She _______ before.
A.hasn't flown
B.didn't fly
C.hadn't flown
D.wasn't flying
555.答案是D;前半句为过去的过去，表示当时在进行；后半句为现在的情况，故选用单数形式。
556.答案是C;hadn't flown动作发生在nervous之前。
