The answer is faults because when divergent boundaries occur, the tectonic plates move away from each other, therefor creating a fault in the earths crust.
I am pretty sure that this question belongs in the maths section not the geography section.
Anyways, the answer is simply to find.
First make sure the units are the same.
Secondly, subtract the first sphere number by the second.
The answer is 35 cubic in.
Hope I helped!!! :)