Besides using environmental （），many seasonal migrators probably have an inborn “calendar” that tells them when to migrate．
A.正确答案Dindexes
B.symbols
C.signs
D.clues
Besides using environmental （），many seasonal migrators probably have an inborn “calendar” that tells them when to migrate．
Besides using environmental （），many seasonal migrators probably have an inborn “calendar” that tells them when to migrate．
A.正确答案Dindexes
B.symbols
C.signs
D.clues
