There are many ways for us to reduce (减少) waste. For example, we can reuse paper and plastic bags. We can also recycle newspapers, glass, and cans (易拉罐)  1new things. Here are some useful 2 to reduce waste.
   Write on both sides of paper
   Many people write on only one side of their paper. Why not try using both sides? Some students only use half the pages of their 3 before getting new ones. Try to use up all the paper. People often give cards on birthdays or festivals. How about trying to 4 e-cards (电子贺卡) instead?
   Don’t throw away your old batteries (电池)
   Maybe you do not know that old batteries make the soil 5. This is because they have poisonous (有毒的) parts in 6. So, don’t throw away batteries. 7 them together. Try to use rechargeable (充电的) batteries for your games and CD players.
Give 8 all your unwanted clothes
   There are many poor families in China. You may ask your mother to keep your unwanted clothes. Then you can pass them on to those poor children. You will feel happy that you are 9 someone else. Also, you can see  10 your friends or relatives (亲戚) want them before throwing them away.

在Rt△ABC中，∠C=90°，∠A=30°，AB=4，则AC=____．

人体的密度跟水的密度差不多。小雨同学身体质量为50kg，试估算他身体的体积。