They've kept up a friendship with the teacher whom they were at school twenty years ago.
A. with
B. about
C. since
D. till
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(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.