The word "random" (Line 3, Para. 3) is closest in meaning to______.

A．birds prepare for breeding
B．bird feathers differ from species
C．birds shed and replace their feathers
D．birds are affected by seasonal changes

A．Although birds preen, the feather wears out.
B．Two processes are involved in molting.
C．When most feathers have been replaced, then the molt is completed.
D．A bird seems unwell when molting.