A bag of marbles contains: 5 red, 4 blue, 6 green and 2 yellow marbles, two marbles are pulled from the bag, one at a time and not replaced. What is the probability of drawing:

a. A red then a green marble.

b. Two green marbles

c. A blue marble given that a yellow marble has already been drawn.

d. A yellow marble given that a yellow marble has already been drawn