One day an American called Simon went to London to visit his friend, Rick. Rick told him that his flat was on the first floor. When he arrived, Simon went straight to the first floor of the building. But he was told that there was no Rick on that floor. Do you know why?

    In fact, the British call the first floor of a building the ground floor. The floor above the ground floor is the first floor, while Americans would call it the second floor.

    The story shows that there are a few culture differences between Britain and America, though the British and Americans both speak English.

    The British usually hide their feelings. They seldom start a conversation with strangers. For example, on the train the British often spend their time reading newspapers or books. But Americans are quite different. They're more active and easier to talk with.

    The British and Americans may use different terms for many things. The British usually use “football”, “eraser” and “mail” while Americans prefer to use “soccer”, “rubber” and “post”.

一个盒子中有4个除颜色外其余都相同的玻璃球，1个红色，1个绿色，2个白色，现随机从盒子中一次取出两个球，这两个球都是白球的概率为(    )