选词填空。

excited, skill, may, they, without, but, include, part

    American teenagers(青少年)enjoy doing part-time jobs．They think it's an important____of growing up．They can learn some important____by doing part-time jobs and they can also make some money in this way．
    American teenagers usually begin to do part-time jobs at the age of 14．On weekdays，they____go to work a few hours a week after school．They usually work for longer at weekends．
    In summer,American teenagers are____if they find part—time jobs at camps or swimming pools．These jobs can bring____much fun．
    Some difficult jobs____teaching， nursing and police work．____these jobs can be helpful for their future．When they grow up，the students with work experience are more likely (可能的)to be successful than those____work experience．

下图协议的签署表明我国（    ）

把63表示成n个连续自然数的和，试写出各种可能的表示法：　____ ．

已知：x＝3是关于x的方程3x－2a=1的解，则a的值是____．

有下述说法：①是的充要条件.
②是的充要条件.
③是的充要条件.则其中正确的说法有（   ）