A bag contains 1 blue marble, 2 red marbles, and 7 yellow marbles. The probability of drawing a blue marble out of the bag is one-tenth or 10%. How many of what color of marbles must be added to the bag so that the probability of a blue marble being drawn at random from the bag is 75%?
A) There must be 7 red marbles added to the bag.
B) There must be 28 blue marbles added to the bag.
C) There must be 21 yellow marbles added to the bag.
D) There must be 35 marbles added to the bag.