下列短文，从短文后各题所给的A、B、C、D 四个选项中，选出可以填入空白处的最佳选项。

    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!