Let $c_1, c_2, c_3, \cdots$ be a series of concentric circles whose radii form a geometric sequence with common ratio as $r$. Suppose the areas of rings which are formed by two adjacent circles are $S_1, S_2, S_3, \cdots$. Which statement below is correct regarding the sequence $\{S_n\}$? A) It is not a geometric sequence B) It is a geometric sequence and its common ratio is $r$ C) It is a geometric sequence and its common ratio is $r^2$ D) It is a geometric sequence and its common ratio is $r^2-1$