X线球管焦点大小常以1.0、0.6等值标注。其值称为（）
A.主焦点
B.副焦点
C.实际焦点
D.灯丝长度
E.有效焦点标称值
参考答案：E
Lin's parents are disabled. He________help them with the housework after school every day. [ ]
A. need to
B. need
C. needs to
D. needs
