PM2.5是指大气中直径小于或等于2.5微米的颗粒物，2.5微米等于0.000 002 5米，把0.000 002 5用科学记数法表示为（     ）

如图所示是以质点P为波源的机械波沿着一条固定的轻绳传播到质点Q的图形，则质点P刚开始振动时的方向和波的性质，下列判断正 确的是(　　)

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    Everyone needs friends. We all like to1close to someone. It is nice to have a friend to talk, laugh and do things with. 2, sometimes we need to be alone. We don't always want people 3. But we would feel lonely if we never had a friend.
    No two people are4 Friends sometimes don't get on well. That doesn't mean 5they no longer like each other. Most of the time they will make up(言归于好)，and become 6again.
    Sometimes friends move away. Then we feel very 7. We miss them very much, but we can8 them and write to them. It could be possible that we could even see them again. And we can also 9new friends.
    There's more good news for people who have friends. They live longer than people who don't have. Why? It could be that they are 10. Being happy helps you stay well and it could be good just knowing that someone cares about you.

在人的消化系统中，既有消化又有吸收功能的器官是（  ）