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 free fall acceleration will the 0.50-kg object experience when transported to a distance of 2.0 × 10^6 m from the surface of this planet?
选项：

A:90 m/s^2;
B:20 m/s^2;
C:13 m/s^2;
D:18 m/s^2;
E:28 m/s^2;
F:38 m/s^2; G:48 m/s^2