Mark has a bag that contains 3 black marbles, 6 gold marbles, 2 purple marbles, and 6 red marbles. Mark adds a number of white marbles to the bag and tells Susan that if she now draws a marble at random from the bag, the probability of it being black or gold is \frac37. The number of white marbles that Mark adds to the bag is
Answer Choices
A. 5
B. 2
C. 6
D. 4
E. 3