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