事务是数据库进行的基本工作单位，如果一个事务执行失败，且已经做过更新被重新恢复原状，好像整个事务从来没有过更新，这样就保持了数据库处于 【12】 状态。

Read and write.

1. He        _ (visit) the Great Wall last year.

2. We        _ ( have) a good time yesterday.

3. We often        _(go) to school by bus last year.

4. I           (live) in the village when I was a child.

5. Mike          (see) a big tiger in the nature park last year.    

6. Sam          (do) the housework yesterday.