题目内容：

    School backpacks are always a hot topic in China. Some Chinese parents buy hardcover school backpacks from Japan, although they can be as expensive as 5,000 yuan, according to Chinese newspaper Urban Express. These backpacks are popular because the hard cover helps hold books in place. This can help kids keep their balance and preserve their spines (脊柱). But it turns out that the bags can be hardly fit in the students' desks.

A good backpack is important for children's health. "It can make pressure less on the shoulders," says Dr. Rob Danoff from the US. He also says students need a backpack that's made for the child's size. Cushioned (加垫的) backs and shoulder straps (带子) will help them avoid pain.

However, the biggest problem for school backpacks is that they are too heavy, noted Hou Weiguang, a doctor in Chengdu. "In China, students have many textbooks and exercise books. The weight makes many kids suffer from side-bending (侧弯的) spines," Hou said. He suggests that a backpack should be no more than 15 percent of a child's body weight to avoid pain or injury. That means if a student weighs 50 kg, the backpack should be less than 7.5 kg.

Parents need to make sure their kids only carry what they need. Schools and governments need to help too, added Zheng Yongshun, an education official from Hong Kong. He said schools should offer locker (储物柜) space and shared books so kids can carry fewer books.

1.The underlined word "preserve" in the passage means "________".

A.invent B.protect C.refuse D.improve

2.According to Hou, how heavy should be a backpack?

A.Less than 7.5 kg. B.No more than 50 kg.

C.Less than 15 percent of a child's body weight. D.No more than 50 percent of a child's body weight.

3.The last paragraph is written to offer tips on ________.

A.how to choose good backpacks B.how to reduce the weight of backpacks

C.how to encourage students to share books D.how to design lockers for students