My sister didn't go to the party last night because she ________ the baby for her friend until 9:30.A.must have looked afterB.would have to look afterC.had to look afterD.should have looked after
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C解析该句由because引导的主从句相成，前句表示后果，后句表可原因，与前句时态照应，所以用一般过去时。