According to the theory of isostasy, which statement must be true?
a. the asthenosphere has the same density as the lithosphere.
b. the lithosphere is denser than the asthenosphere.
c. the asthenosphere is denser than the lithosphere.
d. the lithosphere has the same density as the core

According to the theory of isostasy, the statement that correct is C.

Explanation:

Ok, let's first explain what the isostasy is: the Isostasy is the ideal theoretical balance of all large portions of Earth’s lithosphere as though they were floating on the denser underlying layer, the asthenosphere, a section of the upper mantle composed of weak, plastic rock.

The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. And the lithosphere is the solid, outer part of the Earth. The lithosphere includes the brittle upper portion of the mantle and the crust, the outermost layers of Earth’s structure.

Taking into account these characteristics, the lithosphere is less dense than the asthenosphere. This is because less dense substances (lithosphere) will float on top of denser substances (asthenosphere). Since the lithosphere is on top of the asthenosphere it must be less dense.

d. the weight of the book

"d.) carbon dioxide and hydrogen sulfide"