It was raining on the day they arrived.A. in which B. on thatC. on whichD. where

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A. in which
B. on that
(答案→)C. on which
D. where
解析：考察定语从句。表示在具体的某一天介词只能用on，当引导词谓语介词后指物时，引导词只能用which，先行词为the day，故选C。