4000米＋2千米=____千米

如图是河蚌的结构示意图，请据图回答下列问题．

    It's easier to go down a hill than to climb up a hill, so it is easier to have bad habits than to have good ones.

    Bad habits do not come 1. They come little by little while people do not notice their danger. Schoolboys pick up little bad habits in schools and in the streets. When they can't finish their 2 , they copy from their classmates. If they see older boys 3 , they also want to learn to smoke. When they grow older, their habits become 4 strong that they can no longer get rid of (除掉) it. From copying, they fall behind, and smoking is 5 for their bodies. At last they become worse and worse. How necessary it is that we get rid of the bad habits in the beginning!

刘晨生性有些孤僻，平时很少与同学来往。有一次，刘晨因不小心跌伤住进了医院。在住院期间，班里的同学纷纷来医院探望，并祝她早日恢复健康。很快，刘晨康复返校，并逐渐变得活泼开朗起来。对此，下列说法正确的是（    ）