It wasn’t until a month later I received his letter.A. thatB. sinceC. whenD. as

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(答案→)A. that
B. since
C. when
D. as
解析：：考查强调句型。本题是强调句型，是对not until a mouth later 进行强调，故A正确。句意：直到一个月后，我才收到一封信。故A正确。