Anand deposited Rs 6000 on simple interest. He with- draw Rs 4000 and its interest from that amount after 2 years. After next 3 years, he withdraw the rest of the amount and its interest accrued till that time. In all he obtained Rs 900 as interest. The rate of interest per annum was 

(a) 3% 

(b) 4% 

(c) 5% 

(d) 6%