Hello, Pls help me with the below argument

Question

Ever since the iron smelting plant moved in next to the Willowfield river, the fish population has declined precipitously. Environmental advocates claim that the decrease in the fish population is due to iron smelting byproducts that have been released into the river.

Which of the following, if true, would most seriously weaken the environmental advocates' argument?

(A)The population of fish in the nearby Yellowstone river has seen a rise in its fish population over the same period.
(B)The majority of iron smelting byproducts are relased into the air.
(C)The fish population had declined in the year before the iron smelting plant arrived.
(D)Iron smelting byproducts have been found to be harmless to many living creatures.
(E)An invasive fish-eating species of turtle was introduced into the Willowfield river around the time the iron smelting plant moved in.

Thanks
Manju