PT 60 S3 Q11

A mass of “black water” containing noxious organicmaterial swept through ... ...

Which one of the following is an assumption required by the argument?
(A) Masses of black water such as that observed last summer come into the bay more frequently than just once every two centuries.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Complete Section: LR-B, June '10 LSAT (PT 60 Sec. 3)