语法与词汇 Ensuring an adequate water supply _____ a concern ever since people began to live intowns and cities.
【正确答案】 C
【答案解析】句意：自从人们住在城镇里，确保足够的水供应就成为一项重要的事情。Ever since自从……以来，从句用一般过去时，主句用现在完成时。