根据句意及汉语提示，填写单词。

1. I saw Jack sitting       (在……旁边) Jim talking.

2. I don't understand the meaning of this     (符号).

3. My sister got the answer to the      (谜语) very quickly.

4. Many people are against     (做实验) on animals.

5. He gave me a     (恰当的) example, so I understood it easily.

（）不是天然气火灾产生的有毒物质。

A、CO₂

B、N₂

C、SO₂

D、NO₂