問題詳情：
Some scientists believe birds sing some of the time just because they are happy. However, they sing most of the time 61 a very different reason. Their singing is 62 (actual) a warning to other birds to stay out of their territory.
Do you know what a “territory” is? A territory is an area 63 an animal, usually the male, claims as its own. Only he and his family are welcome there. No other families of the same species are welcome. Your yard and house 64 (be) your territory where only your family and friends are welcome. If a stranger should enter your territory and threaten you, you might shout. Probably this would be enough 65 (frighten) him away.
If so, you have actually scared the stranger away without 66 (have) to fight him. A bird does 67 same thing. But an outsider 68 (expect) almost any time, especially at nesting season. So he is screaming all the time, whether he can see an outsider or not. This screaming is 69 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 70 (they) territory.
【回答】
61. for 62. actually 63. that/which 64. are 65. to frighten
66. having 67. the 68. is expected 69. what 70. their
知識點：語法填空
題型：未分類
