air is everywhere around us．hot air is very （l ）．and some early （b ）used hot air．you have seen hot （s ）rising overa fire．the hot air takes it up．some ballons carried the fires under them，then they took up （i ）the sky．it was not a （s ）from of travel．
參考答案:light 轻
balloons 气球
smoke 热烟
into 到天空中
safety 安全设施
