Tom was disappointed that most of the guests ________ when he ________ at the party．
A.left；had arrivedB.left；arrivedC.had left；had arrivedD.had left；arrived在线课程D
when引导的状语从句用一般过去时，故先排除A和C项。再由was disappointed，这一时间可知，客人离开是在其之前。故离开应用过去完成时态。