If \[f(r)=\pi {{r}^{2}}\], then \[\underset{h\to 0}{\mathop{\lim }}\,\frac{f(r+h)-f(r)}{h}=\]

• A.                 \[\pi {{r}^{2}}\]
• B.                 \[2\pi r\]
• C.                 \[2\pi \]
• D.                 \[2\pi {{r}^{2}}\]