句型转换。

1. They did their homework last night. (改为否定句)

    They __________ their homework last night.

2. I was born on June 22, 1984. (就划线部分提问)

    ___________ born?

3. She went to bed at nine last night. (就划线部分提问)

   __________ she __________ to bed last night.

4. We lived in London for about three years. (就划线部分提问)

    __________ you __________ in London.

5. She got there at 8:00.I got there at 8:00, too.(合为一句)

    __________ got there __________ time.

现有H、S、O、N四种元素，请选用其中的元素写出符合下列要求的化学式各一个：

（1）相对分子质量最小的氧化物______；

（2）空气中含量最多的气体______；

（3）一种能形成酸雨的气体______；

（4）标准状况密度最小的气体______．