Energy is the ability to do work.<span />
Answer:
Metals have more mass per unit of volume than wood.
Explanation:
Density is defined as the amount of matter contained in a unit of volume. A material that is denser than another will therefore have more mass per unit of volume.
The density of a body can affect buoyancy, bodies with low enough densities can float in fluids. For example wood can float in water while metals can't.
THere is a standard relationship that gives this result where the capacity of the capacitor is used:

.
We know though that Q/c=V and thus we can use the relationship:
E=Q*V/2 where we have just substituted in. If we also take into account that Q=VC, we can also get that E=V^2*C/2.
We are given the charge and the potential, so the best expression to use is the middle one.
Substituting, we get that E=1/2*8*10^(-10)*20=8*10^(-9).
The answer is B
Lifting weights, did this yesterday :))