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Birds don't fly high up in the sky. The air is too thin. It is hard for birds to breathe (呼吸) іn thin аіr. Тhin aіr dоеѕn't hоld thеm uр.
Birds fly near the ground so that they can see where they are. They look for places they know. Then they do not get lost. Some birds fly so low over the ocean that the waves often hide them. Many birds fly long distances (距离) in spring and autumn.
1．It is hard for birds to breathe in thin air. （2．Thin air doesn't hold birds up, so they don't fly high up in the sky. （
3．Birds fly near the ground to find food. （
4．Sometimes waves can be higher than some birds fly. （
5．Many birds fly long distances only in spring.（
同类型试题
y = sin x， x∈R， y∈[–1，1]，周期为2π，函数图像以 x = (π/2) + kπ 为对称轴
y = arcsin x， x∈[–1，1]， y∈[–π/2，π/2]
sin x = 0 ←→ arcsin x = 0
sin x = 1/2 ←→ arcsin x = π/6
sin x = √2/2 ←→ arcsin x = π/4
sin x = 1 ←→ arcsin x = π/2
y = sin x， x∈R， y∈[–1，1]，周期为2π，函数图像以 x = (π/2) + kπ 为对称轴
y = arcsin x， x∈[–1，1]， y∈[–π/2，π/2]
sin x = 0 ←→ arcsin x = 0
sin x = 1/2 ←→ arcsin x = π/6
sin x = √2/2 ←→ arcsin x = π/4
sin x = 1 ←→ arcsin x = π/2
