when he opened his bag on the train,he found that he------to put the clean shoes in

问题描述:

when he opened his bag on the train,he found that he------to put the clean shoes in
A forgot B forget C had forgot D would forget选那个那,我选C但是自己说不清楚为什么

C
从上下文看,因为前面是过去时(打开包),这里就应该用过去完成时(忘记带干净鞋子在打开包之前就已经发生了)