根据句意和首字母提示，用单词的正确形式填空。

1. Danny f_______ off his bike yesterday.

2. On my birthday p_______ my friends sing many songs for me.

3. Spring Festival and National Day are important h_______ in China.

4. We sing special s_______ on Christmas.

一般用______来衡量整个生态系统的生产力的高低。