单选题They've kept up a fr

问题补充：

单选题Theyvekeptupafriendshipwiththeteacherwhomtheywereatschool_____twentyyearsago.A.withB.aboutC.sinceD.till

答案：

A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.