Bobby was six years old, and he went to school. There were nineteen other little boys and girls in his class. They often went to parties together.

    When one of the children had a birthday, he or she invited all the other children to come to a party. And at Christmas all the mothers had parties for all the children.

    Bobby like parties very much .He liked the games they played there ---but he liked the nice food more .His favorite food was cakes.

    Before a party, Bobby's mother always asked him not to eat too many cakes, and Bobby always answered ,”I won't ,Mom .”But he always ate a lot and sometimes he got ill .Christmas came again, and again there were a lot of parties. Bobby went to all of them. The last one was on January 6^th.

    One evening before this party, Bobby's mother called the lady who had the party, Bobby always eats too many cakes at parties. Don't worry, answered the lady “At the party, a child can only have three cakes ---and the cakes are small”

    “Oh, good!” Bobby's mother answered happily. On January 6^th, she took Bobby to the party. When it was time to eat ,Bobby took four cakes and began to eat them .”You can only have three cakes ,Bobby！＂said the lady .She asked the boy to put the fourth one back on the plate.

    “I can't,” answered Bobby .I ate that one first”

根据短文 ，回答问题。

下列关于黄河的说法中，正确的是（　　）

记作：

读作：

在光滑杆上穿着两个小球m₁、m₂，且m₁=2m₂，用细线把两球连起来，当盘架匀速转动时，两小球刚好能与杆保持无相对滑动，如图所示，此时两小球到转轴的距离r₁与r₂之比为（   ）