A spring of spring constant $10^{3} \mathrm{~N} / \mathrm{m}$ is stretched initially $4 \mathrm{~cm}$ from the unscratched position. How much the work required to stretched it further by another $5 \mathrm{~cm}$ ?
(A) $6.5 \mathrm{NM}$
(B) $2.5 \mathrm{NM}$
(C) $3.25 \mathrm{NM}$
(D) $6.75 \mathrm{NM}$