The following chart shows revenue and cost figures for a product development effort over a five year period of time. In what year does payback occur?

year	cost	revenue
1	$30k	$0
2	$20k	$52k
3	$10k	$61k
4	$0	$30k
5	$0	$2k

a. year 2

b. year 3

c. year 4

d. there is not enough information to determine when payback occurs