如图，在Rt△ABC中，CD⊥AB，CA=6cm，AD=3cm，则BD=____ cm．

阅读理解，根据短文选择正确的一项。

    One day Mr. and Mrs. White went shopping by car. They stopped their car near a store. They bought a lot of things and they wanted to put the things in the car. But Mr. White couldn't open the door of the car, so they asked a policeman to help them. The policeman was very friendly to help them. Just then a man came up and shouted:“ What are you doing with my car?” Mr. and Mrs. White took a look at the car's number and they were frozen（僵住了） there. It wasn't their car.

写几句保护地球的标语。

如：精心保护地球，造福子孙后代。