单选题He had to___ before 

问题补充：

单选题Hehadto___beforetheyfinishedthecoursebecausetheywerenotwelldisciplined.A.drop inB.drop outC.drop offD.drop by

答案：

B解析这里的短语意思各不相同，drop out 退学,drop in /drop by顺便拜访，drop off 下降，减少。