Most American families are smaller than the families in other countries.
Children in the USA will 1 their parents' home when they grow up. They usually live far from their parents because they often goo to different places to find good jobs. They often  2 to their parents or phone them, and they can choose their own 3.  Americans think it's important for young people to make their own decisions.
    Children are asked to do some house work at  4. In many families, children are paid for 5 the housework so that they can learn how to manage their own money.

将新鲜马铃薯切成粗细相同的5cm长条，再将它们分别放在浓度不同的甘露醇溶液中，4h后测量每条的长度，结果如图所示。以下有关分析正确的是（ ）

①马铃薯细胞液的浓度约为30g·L^-1；
②当溶液浓度为40g·L^-1时，细胞开始发生质壁分离；
③在溶液浓度为30g·L^-1～90g·L^-1的范围内，细胞壁也有一定程度的收缩；
在溶液浓度为20g·L^-1，细胞能发生质壁分离。

空气污染指数简称API．下列计入空气污染指数的项目是　 ____　．

A CO₂         B CO          C N₂         D　 ____　．

如图，在平面直角坐标系中，正比例函数y=kx（k＞0）与反比例函数y= $\frac{3}{x}$ 的图象分别交于A、C两点，已知点B与点D关于坐标原点O成中心对称，且点B的坐标为（m，0）．其中m＞0．