21.my sister has been studying for a test____a week.A.in B.since C.before D.for

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21.my sister has been studying for a test____a week.A.in B.since C.before D.for

21.my sister has been studying for a test____a week.A.in B.since C.before D.for
21.my sister has been studying for a test____a week.
A.in B.since C.before D.for

21.my sister has been studying for a test____a week.A.in B.since C.before D.for
D
since后要加ago
应该是for

选B 前面有has been 是现在完成时 用since引导

D
since后要加ago
完成时用for加一段时间做时间词

D

21. D 表示一段时间用for