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.

人体受到某种抗原的刺激会产生记忆细胞，当其受到同种抗原第二次刺激时[ ]

A．记忆细胞的细胞周期持续时间变短，机体抗体浓度增加

B．记忆细胞的细胞周期持续时间变长，机体抗体浓度增加

C．记忆细胞的细胞周期持续时间变短，机体抗体浓度减少

D．记忆细胞的细胞周期持续时间不变，机体抗体浓度减少