The difference between C.I. and S.I. on an amount of $20,000 for 2 years is $72. What is the rate of interest per annum?  Choices:
A) 5%	B) 6%	C) 7%	D) 8%	E) 9%
$72 is the interest on the first year of interest.   Let x be the interest rate.   The interest after the first year is 20000*x.   The interest on the first year's interest is 20000*x*x   20000*x^2 = 72   x = 0.06   The answer is 6%.
<result>B</result>
The difference between C.I. and S.I. on an amount of $20,000 for 2 years is $72. What is the rate of interest per annum?