Which choice could be modeled by a linear function? A. The amount, y, of radioactive material remaining after x years when decay occurs at rate 30% each year.

B. The amount of money, y, In an account after x years earning 4% interest compounded annually.

C. The height, y, of a ball after bouncing x times, if each bounce reaches 2/3 the previous height.

D. The monthly cost, y, to use a cell phone for x minutes at a rate of 4 cents per minute.