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（1）若所抽取的20件乳制品中，等级系数为4的恰有3件，等级系数为5的恰有2件，求a，b，c的值；
（2）在（1）的条件下，将等级系数为4的乳制品记为
，等级系数为5的乳制品记为
，现从这5件乳制品
中任取两件（假定每件乳制品被取出的可能性相同），写出所有可能的结果，并求这两件乳制品的等级系数恰好相同的概率
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阅读下面短文，从方框中找出每段的中心句，与每一段落匹配起来。
Maybe you think of yourself as a good friend, but are you really? If you’re not sure, it’s time to think about what makes a true friend.
1.A good friend is busy but still manages to make time for your friends. He does not see friendships as a waste of time, or something to pay attention to only when he needs a favour.
2.Always be ready to listen to what the other person is telling you, even if sometimes it makes you uncomfortable.
3.Nothing ends a friendship faster than abandoning （抛弃） a friend in need.
4.When it seems like no one in your life will tell you the truth, your friend will have your back.
5.A true friend doesn’t gossip（说闲话） behind your back. When he doesn’t agree with you, it’s done in a respectful way that is meant for you both to come to an understanding.
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Be respectful. Be ready to listen. Be honest. Be there when your friend needs help. Make time for friends. |
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