读“某日昼夜半球示意图”、“太阳直射点移动示意图”，回答下列问题．

下列函数中是反比例函数的是（   ）

已知函数f（x）=x³﹣（k²﹣1）x²﹣k²+2（k∈R），若过函数f（x）图象上一点P（1，a）的切线与直线x﹣y+b=0垂直，求a的值．

某同学制作的DNA双螺旋结构模型中，在一条链中所用碱基模块A∶C∶T∶G为1∶2∶3∶4，则该模型中上述碱基模块的比应为  （  ）

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．