关于水的组成叙述正确的是（　　）

   George liked flying a plane. Once he bought a small plane and learned how to fly it. He soon became so good that he could make his plane do all kinds of tricks（技巧）.
   George had a friend. His name was Peter. One day George said to his friend, “ Peter, would you like to have a trip in my plane?” Peter thought, “ I’ve traveled in a big plane several times, but I’ve never been in a small one, so I’ll go.”
   They went up, and George flew around for half an hour and did all kinds of tricks in a shaken voice（颤抖的声音）, “ Well, George, thank you very much for these two trips in your plane.”
   George was very surprised and asked, “ Two trips? ”
   “Yes, my first and my last.” Answered Peter.

如图，⊙O过正方形ABCD的顶点A、B，且与CD相切，若正方形ABCD的边长为2，则⊙O的半径为（　　）