A spherical planet of mass M and initial radius R is slowly compressed until its radius becomes 21R while its mass remains unchanged. Which statement correctly describes how the gravitational field strength at the planet’s surface changes as a result of this compression?
(a) It becomes twice its original value.
(b) It remains the same as its original value.
(c) It becomes four times its original value.
(d) It becomes one-half of its original value.