Are you carrying too much on your back at school? I'm sure lots of children of your age will say "Yes". Not only the students in China have this problem, but children in the United States also have heavy school bags. Doctors are starting to worry about young students' back and neck because their school bags are too heavy for them. "It's hard for me to go upstairs with my heavy bag," said Rick Hammond, an ll-year-old student in the USA. Rick is among students who have school bags with two straps ,and many other students choose rolling  bags. But even with rolling bags ,getting upstairs is still a problem for children .Doctors say students should carry no more than 10% to 15% of their own body weight  .  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% , because their bodies are still growing. Teachers and parents should do some things to help children. And the best answer is to have less homework! According to the doctor, Scott Bautch, if a child in Grade 5 weighs about 30 kilos, the school bag he carries should not be over  _  .
5 kilos 3 kilos 5.5 kilos 4.5 kilos