When I was trying to find out 1____1_ that prevented so many birds from returning to the north,
问题描述：
When I was trying to find out 1____1_ that prevented so many birds from returning to the north,
When I was trying to find out ____ that prevented so many birds from returning to the north,an unexpected visitor came. A. why it was B. what it did C. what it was D. why it did
答
C 这是一个It was ……that ……的强调句型