AnimAl migrAtion is A sEAsonAl or pErioDiC movEmEnt oF AnimAls in rEsponsE to ChAngEs in ClimAtE or FooD ______, or to EnsurE rEproDuCtion.
A、AltErnAtion
B．AvAilABility
C．utilizAtion
D．CApACity