Why do earthworms come to the surface after a heavy rain?

An earthworm breathes oxygen by absorbing it through its moist skin. It likes damp soil, but if the rain is very heavy, the oxygen in the water-logged soil may run out. Then the worm has to come up into the air to breathe. On top of the wet ground, the worm can breathe and wriggle along.

the answer is "a.) well–developed vascular system."

explanation: