Consider Two Boxes: a and B

Consider two boxes: A and B. Box A contains 3 Red and 1 Blue marble. Box B contains 3 Blue and 1 Red marble. Simon tosses a fair die. If the die comes up with the numbers 1 - 3 then he will choose Box A, if it comes up with 4 - 6, then he chooses Box B. Once a box has been chosen (and each box has ½ chance of being chosen) , he will reach inside and randomly pick one of the four marbles in that box. Suppose, Simon has picked a red marble. What is the probability that the red marble came from Box B?

A) 1/4.
B) 1/2.
C) 3/4.
D) 1/8.