The boy         the first prize, but he was so careless as to make a foolish mistake.

A．must have gained

B．need have gained

C．could have gained

D．shall have gained

不同支座情况的临界力的计算公式为Pij=，l0为压杆的计算长度;当()时，l0=0.5l。

A.一端固定、一端自由

B.两端铰支

C.两端固定

D.一端固定、一端铰支