People in the United States must wear helmets on their bikes
我们常见的头盔有：
a crash helmet 防撞头盔
a bicycle helmet 摩托车头盔
open face helmets 敞开式头盔
full face helmets 全覆盖型头盔
自行车分类
trail bike 越野自行车
folding bike 折叠式自行车
tandem bike 双人自行车
road bike 公路自行车
toy bike 玩具自行车
racing bike 竞速自行车
mountain bike 山地自行车
