21. —What did you think of _____ president?

—I didn’t care for him at first, but after ______ time I got to like him.

A．the, a

B．/, the

C．the, the

D．/, a

X线成像相关的特性中，"X线能穿透可见光不能穿透的物体，在穿透过程中被穿透物体吸收部分X线"属于（）

A.X线

B.荧光效应

C.感光效应

D.电离效应

E.X线的衰减