Why is it important that rock making up the proposed Zealandia be silica-rich?

a. Silica-rich rocks are lower density and ride higher on the asthenosphere as other continents do.
b. Silica-rich rocks are lower density and ride lower on the asthenosphere as other continents do.
c. Silica-rich rocks are higher density and ride higher on the asthenosphere as other continents do.
d. Silica-rich rocks are higher density and ride lower on the asthenosphere as other continents do.