In General, Which of the Following Statements About Elevated, Compressive

In general, which of the following statements about elevated, compressive orogens is true?

A) Crust was thickened by previous mountain building; as isostatic uplift and erosion proceed, the crust is gradually reduced in thickness.
B) Crust is sucked into the mantle by increased convection under mountain belts.
C) Crust was thinned by extensional faulting in the past; isostatic uplift and erosion cause the crustal root to gradually thicken.
D) Crust is flattened by increased atmospheric pressure over mountain belts.