短语：从方框中选择适当的短语，并用其适当的形式填空，有两个多余项。

（每题1分，共5分）

1. ________________ you’ve got a chance, you had better make full use of it.

2. I think the photos can ________ us ________ the days we spent together in the country.

3. When they crossed the Rocky Mountains, they managed to _____________ some mountain goats.

4. He wrote a long moving poem ________________ those who died in the war.

5. It is not polite to ____________ other people.

在有n个结点的二叉树的llink-rlink法存储表示中，n个结点所含有的2n个指针中，必有 【2】 个为空指针。