A businessman pays an annual bonus to his staff that increases by a fixed amount each year. The total bonus paid in the first 3 years is Rs. 90,000, and the total bonus paid in the first 5 years is Rs. 2,50,000. By how much does the bonus increase each year?
a) Rs. 15,000
b) Rs. 40,000
c) Rs. 20,000
d) Rs. 10,000