Air masses are classified according to their source region: land versus water and latitude of origin. Explain the meaning of each of the following air mass classification letters. P: _, M: _, T: _

a) P: Polar, M: Maritime, T: Tropical
b) P: Tropical, M: Maritime, T: Polar
c) P: Polar, M: Continental, T: Tropical
d) P: Continental, M: Maritime, T: Polar