Many stores run "secret sales": Shoppers receive cards that determine how large a discount they get, but the percentage is revealed only after the purchase has been totaled at the cash register. The store is required to reveal (in the fine print) the distribution of discounts available. Are each of these probability assignments possible. Which of these probability assignments are legitimate?

Probabilites
10% off 20% off 30% off 50% off
a. 0.20 0.20 0.20 0.20
b. 0.50 0.30 0.20 0.10
c. 0.80 0.10 0.05 0.05
d. 0.75 0.25 0.25 -0.25
e. 1.00 0 0 0