The students _______ busily when Miss Brown went to get a book she _______ in the office.

A．had written; left

B．were writing; has left

C．had written; had left

D．were writing; had left

曲柄摇杆机构的压力角是（）。

A.连杆推力与运动方向之间所夹的锐角；

B.连杆与从动摇杆之间所夹锐角；

C.机构极位夹角的余角；

D.曲柄与机架共线时，连杆与从动摇杆之间所夹锐角。