They considered          a computer, which was considered       a great help in their work．

A．to buy; to be

B．buying; being

C．to buy; being

D．buying; to be

两手用力鼓掌时，双手都会感到疼，说明物体间力的作用是______的．用力拉橡皮筋，橡皮筋会变长，说明力可以改变物体的______．