正确答案：
D
解析：
Since the probability of selecting a blue marble at random from the jar is 1/3 and the probability of selecting a green marble at random from the jar is 4, the probability of selecting a white marble is 1-(1/3 + 4/9) = 1- (7/9) = 2/9. If x marbles of the 54 marbles in the jar are white, then x/54 = 2/9, 9x = 108, x = 108/9 = 12.
