青蛙的生殖发育特点是（    ）

如图所示，装置竖直放置，上端是光滑细圆管围成的圆周轨道的一部分，半径为R（圆管内径＜＜R），轨道下端各连接两个粗糙的斜面，斜面与细圆管相切于C，D两点，斜面与水平面夹角为53°，两个斜面下端与半径为0.5R的圆形光滑轨道连接，并相切于E，F两点．有一质量m=1kg的滑块（滑块大小略小于管道内径），从管道的最高点A静止释放该滑块，滑块从管道左侧滑下，物块与粗糙的斜面的动摩擦因数μ=0.5，（g=10m/s²，sin53°=0.8，cos53°=0.6），则（   ）

   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 .