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.

从装有若干个大小相同的红球、白球和黄球的袋中随机摸出1个球，摸到红球、白球和黄球的概率分别为 $\frac{1}{2}$ ， $\frac{1}{3}$ ， $\frac{1}{6}$ ，从袋中随机摸出一个球，记下颜色后放回，连续摸3次，则记下的颜色中有红有白但没有黄的概率为（   ）

用如图所示装置进行如下实验，能达到实验目的的是（　　）