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主键约束即对主键进行限制，其值不能为空也不能重复
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Directions: The following paragraphs are given in a wrong order. You are required to reorganize these paragraphs into a coherent article by choosing from the list A—G to fill in each number box. Paragraph B and E have been correctly placed.
Lack of proper nourishment reduces their power to disease.
All the attempts to the child from the burning building were in vain.
The girls was so by the mighty river that she could spend hours sitting on its bank and gazing at the boats and rafts going and coming.
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