147 A mother and her little baby are playing a game. They start from the same point and move along a circular track with a radius of 60 meters in the same direction at speeds of 0.4 meters/second and 0.2 meters/second, respectively. However, there is a cylindrical artificial hill with a radius of 30 meters at the center of the circle (which can block the line of sight). The longest continuous time they can see each other is
A. $100 \pi$ seconds
B. $200 \pi$ seconds
C. $300 \pi$ seconds
D. $400 \pi$ seconds
147 B. It can be assumed that the baby remains stationary while the mother runs away at a speed of 0.2 meters/second. As shown in Figure 5, when the mother runs from $A$ to $B$, they can no longer see each other. It is known that $\angle A O C=\frac{\pi}{3}, \angle A O B=\frac{2 \pi}{3}$, so the length of $\overparen{A B}$ is $60 \times \frac{2 \pi}{3}$. The required time is
$$
60 \times \frac{2 \pi}{3} \div 0.2=200 \pi \text { (seconds). }
$$