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

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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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