171. A ball with diameter 20 cm is placed between a flat mirror and a concave mirror, and its shadow and penumbra are formed on the concave mirror. The radius of the light source is 4 cm, and the distance from the center of the light source to the center of the ball is 30 cm, and the line connecting these two centers is perpendicular to the flat mirror. If the diameter of the ball's shadow is 40 cm, what is the width of the penumbra in centimeters? \[
(1)\ 4 \qquad (2)\ 5 \qquad (3)\ 8 \qquad (4)\ 10
\]
\textbf{171.} A ball with diameter 20 cm is placed between a flat mirror and a concave mirror, and its shadow and penumbra are formed on the concave mirror. The radius of the light source is 4 cm, and the distance from the center of the light source to the center of the ball is 30 cm, and the line connecting these two centers is perpendicular to the flat mirror. If the diameter of the ball's shadow is 40 cm, what is the width of the penumbra in centimeters?
\[
(1)\ 4 \qquad (2)\ 5 \qquad (3)\ 8 \qquad (4)\ 10
\]