He was told that it would be at least three months he could recover and return to work. A

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A. when
(答案→)B. before
C. since
D. that
解析：before引导时间状语从句，意为“在……之前”，表示他康复重返工作还至少三个月时间。when引导时间状语从句，意为“当……的时候”。since引导时间状语从句，意为“自从”，表示从某个时候一直延续到现在，常与现在完成时连用。that用于名词性从句或定语从句。