They've kept up a friendship with the teacher whom they were at school twenty years ago.A

问题补充：

Theyve kept up a friendship with the teacher whom they were at school twenty years ago.

A. with

B. about

C. since

D. till

答案：

(答案→)A. with

B. about

C. since

D. till

解析：易错选B或C, whom在这里指代the teacher,因此定语从句可还原为： They were at school/with the teacher twenty years ago.