Determine if the following are probability models (if no, state why).
a. X 3 6 9 12 15
P(X) 4/9 2/9 1/9 1/9 1/9


b. X 1 2 3 4 5
P(X) 3/10 1/10 1/10 2/10 3/10

c. X 20 30 40 50
P(X) 0.8 0.2 0.9 0.3