用括号中的动词适当形式填空。

1. We are so glad ________ (hear) the news.

2. I saw him ________ (cross) the road safely and ________ (run) away.

3. You can't make him ________ (do) what you want.

4. I don't know where ________ (meet) him.

5. What makes you ________ (think) I'm a farmer?

6. Did you see him ________ (go) upstairs?

7. It is better ________ (put) your money in a bank.

8. It is difficult for the boy ________ (swim) across the river.

什么是生物链？什么是生物富集？并举例对两者加以简单说明。