---Do you still remember when we went to Chong qing?----I can’t remember now but sometime

问题补充：

---Do you still remember when we went to Chong qing?

----I can’t remember now but sometime last summer?

A. could it be

B. might it be

C. must it have been

D. could it have been

答案：

A. could it be

B. might it be

C. must it have been

(答案→)D. could it have been

解析：句意：——你还记得我们什么时候去的重庆？——我现在记不清了，但是可能是不是去年夏天某个时候呢？。对过去事实的推测用情态动词+have done,故排除AB。用于一般疑问句用情态动词could,用于肯定句用情态动词must。故选D。