若复数（a+i）²对应的点在y轴的负半轴上（其中i是虚数单位），则实数a的值是______．

用所给单词的适当形式填空。

1. Mary told me that she had been to China ____ (two).

2. Here are the results of the _____ (student) activity survey at Hilltop High School.

3. Can you tell me who is the _____ (healthy) student in your class?

4. My _____ (eat) habits are pretty good.