Which of the following statements correctly describe George Biddell Airy's theory of isostasy?

I. Airy's model of isostasy is based on the idea that topographical differences on the Earth’s surface are balanced by variations in the thickness of the Earth's crust, similar to icebergs of varying height floating in water.

II. In Airy's theory, the crust has a uniform density, and elevations are supported by the presence of crustal 'roots' that extend deep into the denser mantle beneath.

III. Airy believed that denser crustal sections under mountains compensate for the topographical elevation by being thinner than the crust under lowland areas.

IV. The theory posits that mountain ranges have a significant underlying root of low-density crustal material extending into the mantle, which accounts for their elevation by floating on the denser substrate.