It is rather difficult to make friends with her but her friendship is truer than any o

问题补充：

It is rather difficult to make friends with her, but her friendship, , is truer than any other’s.

A. while gaining

B. after gaining

C. when to gain

D. once gained

答案：

A. while gaining

B. after gaining

C. when to gain

(答案→)D. once gained

解析：由句意可以得知，her friendship与gain是被动关系，省略了it is。