The old lady’s hand shook frequently. She explained to her doctor ________ this shaking had begun half a year before, and ________, only because of this, she had been forced to give up her job.

A．when; how

B．how; when

C．how; how

D．why; why

关于X线管阴极的叙述不正确的是（）。

A.灯丝发射电子的聚焦程度取决于集射罩的形状和灯丝在集射罩的位置

B.灯丝的宽度和长度决定了焦点的形状

C.灯丝一般绕成螺旋管形

D.集射罩与灯丝具有不同的电位

E.阴极由灯丝和集射罩组成