求帮分析这定语从句.John is the only one of the boys who—the piano wel

John is the only one of the boys who—the piano well.A plays B play C playing D are playing
选A
定语从句先行词结构是“one of the +复数名词”,引导词充当主语时,谓语用复数形式；
定语从句先行词结构是“the (only)one of the +复数名词”,引导词充当主语时,谓语用单数形式；
This is one of the books which have been bought by my teacher.(老师买了很多书）
This is the （only）one of the books which has been bought by my teacher.（老师只买了一本书）