单选题
(Thirteen hundred) medical professionals, (all of which) have been trained to treat drug dependency, (attended) the annual convention (sponsored) by a society.
【正确答案】
B
【答案解析】改为all of whom，因为medical professionals(医学从业人员)是人，而非物。句意为：1300名在治疗毒瘾方面训练有素的医疗从业人员出席了由某协会赞助的每年一次的会议。