OG 20，思路偏差，求救求救...

yearyear

about (a)

my two cents: health officials' explanation是说improved water treatment plant 的 sanitary condition，a选项虽说已经建厂，但没说工厂的sanitary condition是否合格。

sunshine365

A不构成削弱，原因：

题干中说
improved sanitary conditions at water-treatment plants 导致今年的水质变好，继而减少了疾病发生。题干中并没有提这些水厂是什么时候建设的，所以什么时候建厂没有关系，关键是什么时候improved sanitary conditions at water-treatment plants。

如果把A改成“Many new water-treatment plants have
            improved sanitary conditions
             in the last five years in the rural county.”那么就构成削弱。

    

            

JJUUHHUU

强，明白了~~