I dont understand how the D is suppose to weaken the arguement? I was thinking about E which sounds like a better fit.
The conclusion is that the increase in seal popullation has led to decrease in Cod popullation, based on the fact that the Seal dont eat cod(thats what I interpret from "negligible role in diet"). SO, if Cod is mostly eating capelin, why should the Seal start eating the Cod? ANd even if that was the case, if the number of Cod was much lesser than the Seal, the Cod's preference for Capelin would not have any significant effect.
E seems like a better weaking statement as it says the reduction of Cod popullation started before increase of Seal popullation...so might be a different cause.
Thanks!