He is one of the students who I’m sure always do best.A. his B. myC. one’s D. their

问题补充：

He is one of the students, who, I’m sure, always do best.

A. his

B. my

C. one’s

D. their

答案：

A. his

B. my

C. one’s

(答案→)D. their

解析：who always do their best是一个非限定性定语从句，修饰students，I’m sure是插入语。