--You know the exact date ________ you posted the letter, don’t you?
--No. But I remember it was a holiday, _______ happened to be Mother’s birthday. A.when; whichB.that; itC.which; thatD.that; when
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A解析第一空date为先行词，在定语从句中作状语，第二空中的which引导非限制性定语从句，指代holiday。