Why is oil the world's main source of energy? 

A)  It has an artificially low cost. 
B)  It has a low net useful energy yield. 
C)  It is environmentally friendly. 
D)  It is relatively expensive.

Net Useful Energy Yield

The amount of energy that remains available for use after subtracting the energy expended in obtaining and converting it.

Oil

A fossil fuel formed from the decayed remains of ancient organisms, used primarily for energy production and as a raw material in manufacturing.