PT 61 S4 Q17

Near many cities, contamination of lakes and riversfrom pollutants in rainwater ... ...

The statement that contamination of lakes and rivers from pollutants in rainwater runoff exceeds that from industrial discharge plays which one of the following roles in the argument?
(A) It is a conclusion for which the claim that water itself should be considered a polluter is offered as support.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Complete Section: LR-B, Oct. '10 LSAT (PT 61 Sec. 4)