Most American families are smaller than the families in other countries. Children in the USA will leave their parents when they grow up. They usually live far from their parents because they want to find good jobs. They often write to their parents or telephone them. And they often go to visit their parents on holidays.

    Parents usually let their children choose their own jobs. Americans think it is important(重要的) for them to make decisions(作决定).

    Children are also asked to do some housework at home. And in many families，children will get money from their parents for doing some housework. Because American parents want to let their children learn how to make money for their own use.

①碳+氧气  $\stackrel{点燃}{\to }$    二氧化碳

②汽油+氧气 $\stackrel{点燃}{\to }$  二氧化碳+水

③生石灰+水→熟石灰

④氢氧化钙+二氧化碳→碳酸钙+水