An object of mass 0.50 kg is transported to the surface of Planet X where the object's weight is measured to be 20 N. The radius of the planet is 4.0 ×10^6 m. What is the mass of Planet X? (G = 6.67 ×10^-11 N × m^2/kg^2)
选项：

A:13 ×10^19 kg;
B:17 ×10^22 kg;
C:9.6 ×10^24 kg;
D:21 ×10^25 kg;
E:22 ×10^29 kg;
F:21 ×10^28 kg