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
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
问题解答：
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