Ensuring an adequate water supply have been a concern ever since people began to

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问题 Ensuring an adequate water supply have been a concern ever since people began to live in towns and cities.

选项

答案 have—has

解析译文：自从人们开始居住在城镇和都市以来，保障足够的水供给一直是个引人关注的问题。
分析：考查主谓一致。主语为动名词短语时，谓语动词应为单数形式，have应改为has。

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