150 In a unit circle, inscribe a square, then inscribe a circle in this square, and then inscribe a square in this circle. Continue this process infinitely. Let $C$ be the sum of the areas of all the circles, and $S$ be the sum of the areas of all the squares. Then $\frac{C}{S}$ equals
A. $\frac{\pi}{2}$
B. $\pi$
C. $\sqrt{2}$
D. $2 \pi$
150 A. Let $r_{n}$ be the radius of the $n$-th circle, and $x_{n}$ be the side length of the $n$-th square, then $x_{n}=\sqrt{2} \cdot r_{n}$, $\frac{\pi r_{n}^{2}}{x_{n}^{2}}=\frac{\pi}{2}$. Therefore, $\frac{C}{S}=\frac{\pi}{2}$.