Joe invested a certain sum of money in a simple interest bond whose value grew to $310 at the end of 3 years and to $410 at the end of another 5 years. What was the rate of interest in which he invested his sum?	Choose the correct choice from the following options:
A) 6 %
B) 7 %
C) 8 %
D) 9 %
E) 10 %

Rationale: "in 5 years , the value grew $ 100 , so the simple interest was $ 20 per year . in 3 years , the total interest was 3 * $ 20 = $ 60 the principal is $ 310 - $ 60 = 250 . the interest rate is $ 20 / $ 250 = 8 % the answer is c ."
Answer: C