While they ________ the bus, an old man fell to the ground.    [ ]

A. are waiting for  

B. wait for  

C. will wait for   

D. were waiting for

Excel中状态栏提供了快速计算区，可以实现（）函数的功能。

A.求和

B.求平均值

C.计数/计数值

D.最大值和最小值