People still don’t know how languages get originated.（)

Many people know the importance of business ethics, but still some people don’t keep to it.（)

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— How are you getting along with your study？ —___.A、Fine. What about you？B、I don’t know.

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I don’t know the origin of this custom.

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I don’t know the origin of this custom.

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I don’t know the 【origin】 of this custom.A.nature

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I don’t know the 【origin】 of this custom.A.nature

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Years ago we didn’t know this but recent science______that people who don’t sleep well soon get il

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