Having checked the doors were closed , and _________ all the lights were off, the boy opened the door to his bedroom.

A．why

B．that

C．when

D．where

一种元素的两种同位素的原子核A和B，以相同速度垂直磁场方向射入同一匀强磁场中做匀速圆周运动．若认为中子和质子的质量是相同的，则它们的轨道半径之比等于　　　

A．中子数之比

B．核子数之比

C．质子数之比

D．以上都不对