单选题 Birds that live on the boundary between two dialects or that spend time in different areas, according to Donald Kroodsma, can become "bilingual," ______.
Ⅰ. ff they are living on the boundary between two dialects
Ⅱ. when they move or stay in different areas
Ⅲ. on the condition that they are thrown together and forced to learn the songs of their neigh-bors' songs