Which of the following tables represents a proportional relationship?

a. x 8 16 24 32
y 24 48 72 96

b. x 8 9 10 11
y 24 48 72 96

c. x 32 33 34 35
y 96 72 48 24

d. x 32 36 40 44
y 96 112 128 144