—Those scientists invented a robot to care for kids!

—It's not surprising. They ________ wonders all the time.[ ]

A. worked

B. were working

C. work

D. have been working

经常烧毁灯泡和分电器触点的故障原因是（）

A．调节器调节电压过高；

B．发电机皮带打滑；

C．充电电流不稳