Two lakes in the Pawpaw mountains, Quapaw and Highwater, were suffering from serious declines in their fish populations ten years ago. Since that time, there has been a moratorium on fishing at Quapaw Lake, and the fish population there has recovered. At Highwater Lake, no such moratorium has been imposed, and the fish population has continued to decline. Thus, the ban on fishing is probably responsible for the rebound in the fish population at Quapaw Lake. Which one of the following, if true, most seriously weakens the argument above? ['There used to be a greater variety of fish species in Highwater Lake than in Quapaw Lake, but there no longer is.', 'Highwater Lake is much larger than Quapaw Lake.', 'Prior to the ban, there was practically no fishing at Quapaw Lake.', 'Highwater Lake is in an area of the mountains that is highly susceptible to acid rain.']