___such heavy pollution already, it may now be too late to clean up the river.

将四个选项带入原句中：
A.Having suffered such heavy pollution already,it may now be too late to clean up the river.
河流经历了如此严重的污染,现在再治理恐怕已为时过晚.
B.Being suffer such heavy pollution already,it may now be too late to clean up the river.
河流正在已经遭到严重污染,现在再治理恐怕已为时过晚.
*使用了正在进行时,然后还出现了“already”,搭配不当
*suffer用法错误,和be搭配应该是suffered
C.To suffer such heavy pollution already,it may now be too late to clean up the river.
河流为了已经遭到严重污染,现在再治理恐怕已为时过晚.
*动词不定式To do表示目的,“为了……”,原句的意思应该不是“为了污染河流”
*“already”一般不和不定式搭配,一般用于完成时的,或者修饰状态的
D.Suffered such heavy pollution already,it may now be too late to clean up the river.
河流已经遭到严重污染,现在再治理恐怕已为时过晚.
***A和D选项：
两个都对,A比D更好,所以选A.
A选项用了Having done,完成时非谓语动词用在这样的句子里面是带有“因果含义”的
也就是说A选项Having suffered such heavy pollution already,it may now be too late to clean up the river.
因为“河流经历了如此严重的污染,”所以“现在再治理恐怕已为时过晚.”
D选项,仅仅是陈述了事实,采用了过去时,说明河流遭到严重污染,与主句“it may now be too late to clean up the river.”之间并没有强烈的因果关系.
所以从这一角度来讲,A比D更好.