Isaac keeps track of the amount of money in his savings certificate each month. He has accumulated the following data:

(0,50); (1,55); (2,60.5); (4,66.55)

What is the common difference or ratio?

a. The common difference is 0.48
b. The common difference is 0.50
c. The common ratio is 0.98
d. The common ratio is 1.1