Which of the following situations form a proportional relationship?
a.5 DVDs for $9 and 15 DVDs for $27
b.6 cups of coffee for $8 and 9 cups of coffee for $14
c.3 purses for $60 and 5 purses for $90
d.12 cans for $3 and 30 cans for $7.50
e.2 ink cartridges for $25 and 4 ink cartridges for $75