A spaceship is moving straight toward Earth with its engine turned off. Let M and m be the masses of the Earth and of the spaceship, respectively, with R the distance from the spaceship to the centre of the Earth. As a result of the gravitational force of the Earth, in moving from position 1 to position 2, the kinetic energy of the spaceship increases by
(a) R1R2GMm(R2−R1)
(b) R1R2GMm(R1−R2)
(c) R12R22GMm(R2−R1)
(d) R12R22GMm(R1−R2)