A satellite is moved from a circular orbit of radius r1 to a higher circular orbit of radius r2(r2>r1) without atmospheric drag. Which statement about the external work W that must be done on the satellite is correct?
(a) W is positive and equals the increase in its gravitational potential energy only.
(b) W is zero since the gravitational force is conservative and therefore does all the work.
(c) W is positive and equals the increase in its total mechanical energy.
(d) W is negative and equals the decrease in its total mechanical energy.