The figure below shows the construction used to obtain a square with an area equal to that of a given rectangle.
ABCD is a rectangle, HDFG is a square, semicircle with center O
$$A ( ABCD ) = A ( HDFG )$$
The F vertex of the square HDFG in the figure lies on the semicircle with center O.
Given that the perimeter of rectangle ABCD is 36 cm, what is the diameter of the circle in cm?\\
A) 12\\
B) 15\\
C) 18\\
D) 21\\
E) 24