A balance is used to determine the mass $m$ of several cylindrical metal rods of length $l$. Each of the metal rods has the same radius $r$. The graph shows how $m$ varies with $l$. The gradient of the graph is $G$. Which expression gives the density of the metal?
- A$\frac{G}{2\pi r}$
- B$G2\pi r$
- C$\frac{G}{\pi r^2}$
- D$G\pi r^2$