根据句意及首字母提示，写出空白处所缺的单词。

1. When the t_______ lights are red, you must stop and wait.

2. Did you r_______ her letter the day before yesterday?

3. We must keep our classroom c_______ every day.

4. Look! It's raining outside. You'd better put on your raincoat or take an u_______.

5. N_______ Li Lei nor Liu Mei is in the classroom now.

砷酸钾,砷酸二氢钾,砷酸镁