题目内容：
31. My wallet is nowhere to be found.I_____it when I was on the bus.
(A) must drop
(B) should have dropped
(C) had dropped
(D) must have dropped
参考答案：
选择题：31.My wallet is nowhere to be found.I_____it when
- 题目分类：郑州工程技术学院-大学英语（2）.
- 题目类型：选择题
- 号外号外：注册会员即送体验阅读点！
My wallet is nowhere to be found.I_____it when I was on the bus.
(A) must drop
(B) should have dropped
(C) had dropped
(D) must have dropped