A student while walking on the road observed that a cloud of black smoke belched out from the exhaust stack of moving trucks on the road. Choose the correct reason for the production of black smoke:

[A] Limited supply of real leads to incomplete combustion of fuel.

[B] Rich supply of air leads to complete combustion of fuel

[C] Rich supply of air leads to a combination reaction

[D] Limited supply of air leads to complete combustion of fuel