关于X线管焦点叙述错误的是（）

A.灯丝电子撞击的面积为实际焦点

B.实际焦点在像面上的投影为有效焦点

C.实际焦点由主焦点和副焦点构成

D.有效焦点为圆形

E.有效焦点标称值没有单位

--- I think we have met the word before.

　　　　--- Yes. It is in the reading material ____ we ____ reading yesterday.

A．that; did

B．that; were

C．when; were

D．when; did