Why is the oceanic crust different than the mantle?

Layers that are less dense, such as the crust, float on layers that are denser, such as the mantle. ... Because continental crust is less dense than oceanic crust it floats higher on the mantle, just like a piece of Styrofoam floats higher on water than a piece of wood does.

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The correct answer is a)  a resource with a limited supply; it will run out.

The answer is c, i, iii, and iv. this is because farming and mining deal with harvesting raw materials, construction work falls under manufacturing, and piloting a plane is a service. ii is wrong because making bread is using  raw materials to produce goods, which would fall under the secondary sector, not primary.