seriously weaken的题都做不对

Civic Leaders: The high cancer rate among our citizens is the result of hazardous material produced at your plant.

Board of Directors: Our statistics show that rates of cancer are high throughout the valley in which the plant is situated because local wells that supply drinking water are polluted, not because of the plant.

Which of the following, if true, most seriously weakens the board’s claims?

(C)  Cancer-causing chemicals used at the plant are discharged into a nearby river and find their way into local wells.

(E)  Some of the pollutants cited by the board as contaminating the local wells have been present in the wells for decades.

我的理解是Board of Director认为造成high cancer rate的原因不是他们的plant 发放出来的有毒物质,而是提供饮用水源的local wells被污染了。那么要削弱，为什么不选E（local wells的污染早就有了）,而去选Ｃ呢？我觉得Ｅ更好啊。

E明显是支持Board的.

Leader说:我们市民的高致癌率都是因为你们工厂的有害物导致的.

Board辩解的理由是:什么呀,不是我们工厂.我们的统计表明癌症发病率是在我们工厂坐落的valley里面,而这里面供水的井都被污染了,不是因为我们工厂.

哪个可以削弱Board的话?

C选项的意思:工厂里用的容易导致癌症的化学物被排放到周边的河里,又跑到了当地的井了.(意思是,还是Board工厂的致癌化学物污染了井水,).

E:Board说的有害物质早以存在多年了.(意思是说，不是我们工厂排的,是早已经存在了.明显是支持Board的)

所以E不对.