根据句意和汉语注释，写出单词的正确形式。

1. We all know ________ air does harm to our health. It's necessary to keep our air fresh and clean. (污染)

2. Be careful! This kind of disease can be easily ________ everywhere by some animals. (传播)

3. It was very ________ to hear such a strange noise at midnight. (恐惧的)

4. The shop owner rose to his feet ________ to run after the thief. (立即)

5. The teacher's serious look reduced the little boy who had just done something wrong to ________. (沉默)