关于and和“，”连接句子的问题。

The traffic rule says young children（ under the age of four and____less than 40 pounds）
must be in a child safety seat.

从句中主语是
young children which is under the age of four and .weighing less than 40 pounds

主语是young children 和weigh存在主动关系，所以用ing和and没有关系，and只是连接句子，只是句子主语省略在前面。
D就是是非谓语动词

若是两句句子
l am a student，and he also is a student
连接两句句子，用（，and）。或者（;)或者直接（。）

and连接的是“under the age of four ”和“weighing less than 40 pounds ”两个修饰“young children ”的定语。says后跟的整个是宾语从句，从句中“young children ”是主语，“under the age of four and weighing less than 40 pounds ”是定语修饰主语，从句的真正谓语是“must be ”，“ in a child safety seat”是介宾短语作be的表语。

weight没有被动语态，排除AC.to do题目的to weight是表将来的，因此用D表示称起来少于40rounds