There is a ball in a box and three kids are guessing what colour it is.  Val says: "The ball is white."  John says: "The ball is blue."  Elvis says: "I agree with Val."  They open the box and find only one of them guessed right.  What colour is the ball? 
选项为:A:white  B:blue  C:Uncertain  请回答其中一个选项.