A spacecraft measures the magnitude of the gravitational force exerted by a planet when it is at three different distances: R, 2R and 3R from the planet's centre. Let the corresponding forces be F1, F2 and F3 respectively. Which option gives the correct ascending order of these forces?
a) F1<F2<F3
b) F1=F2=F3
c) F3<F2<F1
d) F2<F3<F1