X线的产生，下述哪项不正确()

A．需要有自由电子群

B．电子群高速由阴极向阳极行进

C．绝大部分(99%以上)动能转变为X线

D．高速电子流突然受到阻挡

E．同时产生了大量的热能

If you see someone fall overboard from a vessel，you should（）.

A.immediately jump in the water to help the individual

B.call for help and keep the individual in sight

C.run to the radio room to send an emergency message

D.go to the control room for the distress flares