I think it must have been in 1967 ______ I visited the Great Lake the first time.

题目

I think it must have been in 1967 ______ I visited the Great Lake the first time.
A：when
B：which
C：where
D：that

答案

D
是强调句
It must have been.that.
是 It is.that.变化而来(表示肯定的猜测)