A spring of spring constant $5\times 10^3\text{N/m}$ is stretched initially by $5\text{cm}$ from the unstretched position. Then the work required to stretch it further by another $5\text{cm}$ is:

1. A
   $12.50\text{N-m}$
2. B
   $18.75\text{N-m}$
3. C
   $25.00\text{N-m}$
4. D
   $6.25\text{N-m}$