根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。
Have you ever wondered why birds sing? 1 After all, you probably sing or whistle when you are happy.
Some scientists believe that birds do sing some of the time just because they are happy.
2 Their singing is actually a warning to other birds to stay out of their territory. Do you know what a "territory" is? A territory is an area that an animal, usually the male, claims(声称) as its won. Only he and his family are welcome there. No other families of the same species are welcome. Your yard and house are your territory where only your family and friends are welcome. If a stranger should enter your territory and threaten you, you might shout. 3 If so, you have actually scared the stranger away without having to fight him. 4 But he expects an outsider almost all the time, especially at nesting(筑巢) season. So he is screaming all the time, whether he can see an outsider or not. This screaming is what we call a bird' s song, and it is usually enough to keep an outsider away.
Birds sing loudest in the spring when they are trying to attract a mate and warn others not to enter the territory of theirs.
5 Most of it has to do with attracting males and setting up territories.
A．A bird does the same thing.
B．Maybe you thought that they were just happy.
C．Shouting loudly is just like a bird's screaming.
D．You can see that birds have a language all their own.E. Probably this would be enough to frighten him away.
F. However they sing most of the time for a very different reason.
G. So, just like human beings, birds sing for almost the same reasons.