A community group is building a garden on a rectangular lot of land. The lot measures 5x by 10x and the garden will be circular with a radius of 3x. How much of the land will be left over after the garden is built?
A. 41 pi x^2
B. x^2 (50 - 9 pi)
C. x^2 (50 - 3 pi)
D. 41x^2