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

问题描述:

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

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)