The company where I work is not as good as before, so the manager has decided to fire some

employees to ________ the expenses of his company.[      ]

A. cut off  

B. cut away

C. cut up  

D. cut down

对一定量的气体，若用N表示单位时间内与器壁单位面积碰撞的分子数，则 [ ]

A．当体积减小时，N必定增加

B．当温度升高时，N必定增加

C．当压强不变而体积和温度变化时，N必定变化

D．当压强不变而体积和温度变化时，N可能不变