（I）求y=f（x）的解析式；

（II）如果关于x的方程f（x）=a有解，那么将方程在a取某一确定值时所求得的所有的解的和记为M_a，求M_b的所有可能取值及对应的a的取值范围．

函数$f\left(x\right)=e^x-e^{-x}$是(    )

   Some people don’t like doing sports because they think it is hard work and will make____ (他们) feel tired . That’s wrong! Sports help us to____ (保持)healthy , happy and live longer .If you____ (从不) do sports , you will get unhealthy easily . Sports are____ (也)a good way to make friends .People from different____ (地方) may not know each other , but after a game , they often become good friends .And sports in winter and ____ (夏天)are not the same.____ __(swim) is fun in hot weather , while skiing is great____ cold days . Many people____ (享受) watching others play games .They often get very____ (兴奋) when “their” players or teams win .