6. As shown in the figure, a square is divided into 30 rectangles (not necessarily of the same shape) by 4 lines parallel to one pair of opposite sides and 5 lines parallel to the other pair of opposite sides, and the sum of the perimeters of these small rectangles is 30.
Then the area of the original square is ( ).
(A) 1
(B) 4
(C) $\frac{225}{144}$
(D) $\frac{144}{225}$
6. C.

Solve $4 x+2(4+6) x=30$. ( $x$ is the side length of the original square) $x=\frac{15}{12}, x^{2}=\frac{225}{144}$.