The difference between B and D is that D tells us the exact role of the statement. A premise in support of the conclusion.
This differs from B, which claims that pollution from rainwater runoff is a more serious problem than pollution from industrial discharge. Firstly, we are concerned only with contamination of lakes and rivers, but this answer choice draws a much wider conclusion about pollution in general. And secondly, just because something may pollute more than another does not mean that it is more problematic. Imagine that the rainwater runoff is easily filtered, while chemical waste from industrial discharge is highly toxic to wildlife in quantities even 1/10 as high as runoff pollutants.