潜水、登山和宇航都需要用到氧气是因为它能____ ；气焊、炼钢需要用到氧气是因为它能____ ．氮气的化学性质____ ，可用作____ ，氮气约占空气体积的 ____ %．
—It's really difficult to make my dream come true.
— That's true. But you'll succeed if you stick with it.
老师留了16道计算题，小红还要做7道才能完成作业，猜一猜小红已经做了几道？
