—— Do you still remember the day when we went to the Great Wall?

—— I can’t remember it well, but＿＿＿＿＿＿sometime last autumn?

A．might it be

B．could it have been

C．could it be

D．must it have been

活动体验法属于大学教学中常用的研究性教学方法。