A rectangle has length 8 \mathrm{~cm} and width \pi \mathrm{cm}. A semi-circle has the same area as the rectangle. What is its radius?
Answer Choices
A. 4 \sqrt{2} \mathrm{~cm}
B. 4 \mathrm{~cm}
C. 16 \mathrm{~cm}
D. 8 \mathrm{~cm}
E. 2 \mathrm{~cm}