A sum of money invested at compound interest amounts to Rs. 800 in 3 years and to Rs. 840 in 4 years. The rate of interest per annum is :

• A. <table><tr><td rowspan="2">2</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1</center></td><td rowspan="2">%</td></tr><tr><td style="text-align: center;">2</td></tr></table>
• B. 4%
• C. 5%
• D. <table><tr><td rowspan="2">6</td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>2</center></td><td rowspan="2">%</td></tr><tr><td style="text-align: center;">3</td></tr></table>