声音是由于物体的           产生的，在15℃的空气中，声音每秒传播           米．某声源1分钟振动60次，频率为           Hz，是人耳听不到的            ．古代士兵枕着牛皮制的箭筒睡在地上，能及早听到夜袭的敌人的马蹄声．这是因为声音在大地中的传播速度比空气中           （选填“快”或“慢”）

根据句意、英文释义及汉语提示写出单词。

1. Now the air isn't clean in this city. There is air _______ here and there.

2. Your hands look so ________ (脏). Go and wash them.

3. He doesn't study hard, so he often ________ (doesn't pass) in the exam.

4. Please set us a good ________. (榜样)

5. Don't _______ (make fun of) at the weak. We should go to help them.