There is a total of 84 marbles in a box, each of which is red, green, blue, or white. If one marble is drawn from the box at random, the probability that it will be white is 1/4 and the probability that it will be green is 1/7. What is the probability that the marble will be either red or blue?  Choices:
A) 4 / 7	B) 9 / 14	C) 11 / 14	D) 17 / 28	E) 23 / 28

Rationale: "p ( red or blue ) = 1 - p ( white ) - p ( green ) = 28 / 28 - 7 / 28 - 4 / 28 = 17 / 28 the answer is d ."
Answer: D