—Do you know the boy is playing soccer there?—Certainly, he is a friend of my A. who; bro

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(答案→)A. who; brother’s
B. who; brother
C. whom; brother’s
D. whose; brother
解析：定语从句的引导词有关系代词who， whom，whose，which，that， as；关系副词when， where，why。定语从句中引导词的选用主要看先行词在定语从句中作什么成分或者是定语从句中缺什么成分。
1．定语从句中缺主语，修饰人时用who ／that，修饰物时用which ／that引导。
2．从句中缺宾语，修饰人时用 who ／whom ／that或省略引导词；修饰物时用which ／that或省略引导词。
3．从句中缺时间状语，用when或介词＋which引导。
4．从句中缺地点状语，用where或介词＋which引导。
5．从句中缺原因状语或先行词是 reason时，引导词用why ／for which ／ that。
6．从句中缺定语，人和物都用 whose引导。
7．当先行词是way时，其定语从句的引导词用in which ／that。
根据语境可知先行词是the boy是人，故用that或who，又因我弟弟的朋友应用A friend of my brother’s故选择A。