问题
_____ my homework for four hours, but I am not sure whether I can hand it in tomorrow.
A:have done
B:have been doing
C:had been doing
D:had done;
B:have been doing
C:had been doing
D:had done;
正确答案是：
同类题型
The pilot asked all the passengers on board to remain _____ as the plane was landing [ ]
I must return the camera to Cindy. I______it for two weeks.
sa change made by China and the US will witness _____increase in economy, tourism and education.