Which situation is best represented by a non-linear function?

a) Sara gets paid $0.01 for the first hour, $0.02 for the second, $0.04 for the third, and so forth.

b) Darius is willing to pay $1.75 for each baseball card he collects from his fellow collectors.

c) Alfred gets paid $5 an hour for babysitting.

d) Chris pays $188 a month for his new car plus a one-time down payment of $500.