A 1.5×103kg spacecraft is located a distance r from the center of a small moon of mass 7.4×1022kg. The spacecraft–moon system has a gravitational potential energy of −6.0×109J (zero at infinite separation). What is r?
(a) 7.4×105m
(b) 1.2×107m
(c) 1.2×106m
(d) 2.5×106m