一只口袋里装有两种颜色的球，除绿、白颜色不同，其他都相同，两种球的个数一样多，摸到绿球的可能性是多少？如果30个绿球，20个白球，摸到白球的可能性是多少？

    Tom was a farmer. He worked on the farm all day，but sometimes he went to the town market to sell fruit and vegetables. One day, a terrible sound attracted his attention in the town market. He saw a young bull for sale. The bull was white and yellow. It was looking at Tom in fear. Tom walked up and touched its head gently. Just at that time they both seemed to have known each other for a long time. How amazing！Tom bought it at once and called it Amba.

    From then on ， Tom and Amba got on well with each other. But some friends told him that it was dangerous to have such a close relationship with an animal.

    One afternoon， Tom was walking through the forest with Amba. Suddenly， Amba stopped walking and kept pushing Tom with its head. Tom was very surprised and looked around. There was a big snake in front of him. It was beautiful but poisonous. Quickly Amba stepped on the snake's tail with its foot and at the same time Tom picked up a stick and hit the snake's head heavily. Soon the snake died.

    Tom was very grateful for Amba's help. When people heard this, they were shocked at the bull's expression of love for Tom. But for Tom, Amba was not a bull but a member of his family.

曹魏时期，曹丕命陈群选拔“德充才盛”、“有识鉴”之人担任中正，按照“家世、道德和才能”三条标准，选拔“俊秀之士”，一时间“儒雅并进”。由此可知当时（   ）