Most people like bikes. Perhaps you ride a bike to school. Riding a bike can be great of fun.Do you know how to ride a bike safely?

    Careful boys and girls learn to ride their bikes well before they go out on the road.How can you tell whether you are a good rider?

    If you can ride a bike in a straight line,you can ride well .If you can't ,you should practice riding a in a safe place.

    To be safe on a bike, you need to use both hands and both feet. Keep your feet on the pedals(脚蹬)and your hands on the handle bars.

    You know that your bike is only for one person. It's not safe to carry anyone with you. Keep yourself safe when you ride on a bike.Watch where you are going all times.

中国商朝的青铜制作技术非常精湛。目前世界上已经发现的最大的青铜器就诞生在商朝。它是（    ）

直升飞机在空中匀速下降的过程中（忽略燃油的变化），关于直升飞机的动能和重力势能的变化情况描述正确的是（　　）