28. What was unexpected was that the disrespectable fellow ______ the old man.

A．could have robbed

B．should have helped

C．should rob

D．might have helped

在棋类比赛中棋子可以在棋盘上移动但不会掉下来，原来棋盘和棋子都是由磁性材料制成的。棋子不会掉落是因为棋子[ ]

A.受到的重力很小可忽略不计

B.和棋盘间存在很大的吸引力

C.受到棋盘对它向上的摩擦力

D.受到的空气阻力等于重力