He is rather difficult to make friends with but the friendship of his is truer than an

问题补充：

He is rather difficult to make friends with, but the friendship of his, , is truer than any other.

A. once gained

B. when to gain

C. after gained

D. while gaining

答案：

(答案→)A. once gained

B. when to gain

C. after gained

D. while gaining

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