A student discovers a comet heading toward the Sun. The student records the initial distance from the Sun as 'd' while recording the initial mass of the
comet as 'm' which resulted in a gravitational force of 'F'. Further
observation of the comet shows that as it reaches a positon d/2, the mass
of the comet has been reduced to 1/3 of its original mass. Which
expression best represents the value of the gravitational force at d/2?*