The pen he is writing is mine. A. with which B. in which C. on which D. by which

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(答案→)A. with which
B. in which
C. on which
D. by which
解析：. with which是介词+关系代词结构,常用来引导定语从句.with有用的意思,介词之后只能用which,不能用that. with which在定语从句中作状语,即he is writing with a pen.