He is rather difficult to make friends with, but his friendship, ____, is more true than any other.
A. once gained B. when to gain C. after is gained D. while gaining
He is rather difficult to make friends with, but his friendship, ____, is more true than any other.
A. once gained B. when to gain C. after is gained D. while gaining
A
once gained是状语从句的省略, 相当于once it is gained。