\begin{aligned}A &=\frac{1}{4} \pi r^{2} & \qquad & l=\frac{1}{4} \times 2 \pi r \\100 &=\frac{1}{4} \pi r^{2} &\qquad &=\frac{1}{2} \pi r \\400 &=\pi r^{2} &\qquad &=\frac{1}{2} \times \pi \times 11.284 \\r^{2} &=\frac{400}{\pi}&\qquad &=17.7 \mathrm{~cm} \quad \therefore C \\r &=\sqrt{\frac{400}{\pi}} \\&=11.284 &\end{aligned}
