You could use the RND function. It returns a random number between 0 and whatever number you put in it minus 1.
E.g: RND(10) will return a number between 0 and 9.
10+RND(91) will return a number between 10+0 (10) and 10+90 (100).
You can have a number represent each prize. (0 would be prize A, etc.) Make a loop that shows randomly chosen prizes for some time, then stop at one of them.
2そうだね プレイ済み