外力作用的能量主要来自（ ）
A．太阳辐射能
B．重力能
C．地热能
D．核能
答案：A
His movie won several awards at the film festival, _____ was beyond his wildest dream. [ ]
A. which
B. that
C. where
D. it
平面应力状态中，若σy=0，σx≠0，τx≠0，则第四强度理论的相当应力σr4为（）。
A.A
B.B
C.C
D.D
