The largest volcano on Mars rises 27 kilometers above the surrounding plain and covers an area roughly the size of Romania. Even if the Earth' s gravity were as low as the gravity of Mars is, no volcano of such size could exist on Earth, for the Earth' s crust, although of essentially the same composition as that of Mars, is too thin to support even a small fraction of that mass and would buckle under it, causing the mountain to sink. If the statements above are true, which one of the following must also be true on the basis of them? ['The crust of Mars, at least at certain points on the planet, is thicker than the crust of the Earth.', 'On average, volcanoes on Mars are higher than those on Earth.', 'The surface of Mars is less subject to forces of erosion than is the surface of the Earth.', 'The highest volcanoes on Mars occur where its crust is thickest.']