平衡液体中的等压面必为（）
A.水平面
B.斜平面
C.旋转抛物面
D.与质量力相正交的面
参考答案：D
He________ full marks, but he was so careless as to make a spelling mistake.[ ]
A. must have gained
B. might gain
C. could have gained
D. must gain
作用力与反作用力是作用在（）相互作用的物体上。
A、同一个
B、两个
C、三个
