Look! A large number of boys _____ the ground, the number of them _____ 20.  [ ]

A. is running, are

B. are running, is

C. are runing, is

现在人类已经可以控制原子核裂变的速度，我们把核裂变的过程又称作 _________ 反应．利用核裂变提供的能量可以用来 _________ 、 _________ 等．