請(qǐng)指出it在下列各句中的用法.
may i use your dictionary?sorry,i left it at home.
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it is a good idea to bring a camera when we travel around.
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it is polite knocking on the door begor entering a room.
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i found it difficult to protect the environment by oneself.
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it's said that MH370 has been discovered in the Indian Ocean.
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