[解析]
(1)答案:C.细节理解题.根据Not only the students in China have this problem,but children in the United States also have heavy school bags.可知,中美学生的书包都很重.故选C.
(2)答案:A.细节理解题.根据Doctors are starting to worry about young students' back and neck because their school bags are too heavy for them.可知,对孩子的背部和颈部造成伤害,故选A.
(3)答案:D.推理判断题.根据Scott Bautch,a back doctor,said children under Grade 4 should carry no more than 10% of their weight and older children shouldn't stay with over 15%,计算可知,30*15%=4.5,故选D.