A man invested Rs 5000 at some rate of simple interest and Rs 4000 at 1 percent higher rate of interest. If the interest in both the cases after 4 years is same, the rate of interest in the former case is (a) 4% p.a. (b) 5% p.a. (c) `6 1/4%` p.a. (d) `8 1/3%` p.a.