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
网友回答
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。