20. As shown in Figure 4, given rectangle $A B C D, A D=5, C D=$ 3, circles are drawn with centers at $A$, $B$, and $C$ and radii of 1, 2, and 3, respectively. Then the area of the shaded region is approximately ( ).
(A) 3.5
(B) 4.0
(C) 4.5
(D) 5.0
(E) 5.5
20. B.
$$
\begin{array}{l}
3 \times 5-\frac{\pi}{4}-\frac{\pi}{4} \times 2^{2}-\frac{\pi}{4} \times 3^{2} \\
=15-\frac{\pi}{4}(1+4+9)=15-\frac{7 \pi}{2} \\
\approx 15-\frac{7}{2} \times \frac{22}{7}=4.0
\end{array}
$$