你认为，下列哪种做法能够帮助你取得考试的成功（　　）

①相信自己能成功；

②遇到难题绝不放弃；

③专心致志做题；

④能有效地利用周边同学．

A．①②

B．③④

C．①③

D．②④

—How did you like the concert last night ?

—I enjoyed it very much, but the dancers _____ a better job．

A．could have done

B．must have done

C．could do

D．had done