动物在自然界中的作用至关重要，下列叙述不正确的是（    ）

On Christmas Day, children have __  ___ singing, dancing and playing games.

某物体在地面上受到地球对它的万有引力为F．若此物体受到的引力减小到，则此物体距离地面的高度应为（R为地球半径）

从基因文库中获取目的基因的根据是    

阅读理解(D)
    Sharon's teacher told them that people should protect the environment. But to keep it clean, Sharon had to find it first. After breakfast, she went outside to look.
    Sharon met Herman, a squirrel(松鼠). She asked him about the environment. Herman said, "The environment is everywhere you go. The air you just breathed is part of it. The environment is the air, the water, the soil and our food. It's everything around us." Sharon thought and thought. "I think the Sun isn't part of the environment." But Herman said, "Oh，yes, it is. The sun gives you a vitamin(维他命）you need, Vitamin D. So we should protect the environment. It's important for your life. We can't throw rubbish everywhere. We can't waste things anymore. And we should stop polluting the river. And..."
    Sharon said, " Wow, it's a big job to clean the whole environment. I think I will be busy from now on."