Choose the correct statement(s) regarding pollen grains.
I. Pollen grains are the male gametes of the flowering plant.
II. Pollen grain has a prominent two-layered wall.
III. The outer thick wall of pollen grain is called exine and the inner thin wall is called intine.
IV. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic materials known.