如图，已知线段AB=8cm，点C是AB上任一点，点M、N分别是AC和CB的中点，则MN的长度为

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长江全长6300多千米，是我国的第一大河。长江流域中宜昌—城陵矶河段属于荆江河段，容易发生洪水灾害。相关的因素主要是（    ）

①荆江河段流域面积广              ②荆江河段所处的夏季多暴雨的气候特征

③荆江河段河道弯曲，排水不畅      ④长江上游的人类活动

    This is Mike's bedroom. You can see a desk and a bed in the room. On the desk, there is a backpack and three books. One is a Chinese book, the other two are English books. There is a clock on the desk, too. You can see a baseball under the desk. On the bed there is a box. Can you guess what's in the box? Ah, it's a new computer game. It's the present(礼物) for Mike's seventh birthday.