[单选题]
A box of mass $2.0\;kg$ starts to move on a level surface under the effect of a varying force $F$. The variation of $F$ with the displacement $s$ of the object is shown in the graph below.
What is the speed of the box at $s=5\;m$?
A. $5\,m\,s^{-1}$
B. $\sqrt{5}\;m\;s^{-1}$
C. $15\,m\;s^{-1}$
D. $\sqrt{15}\;m\;s^{-1}$
