楼主选A是认为which引导定语从句,但仔细分析下句子主干,our college has been set up in____used to be a small school,明显,has been set up in整个是句子的谓语,in 后面需要带一个宾语,这样句子结构才能完整,故选B what ,引导宾语从句.若选A,则定语从句缺少先行词.