Editorial: Almost every year the Smithfield River floods the coastal f

Though Its very hard to choose b/w 'B' and 'D' but i chose D because as B is already mentioned in the passage(Fish that now feed on those nutrients would start feeding elsewhere. The loss of these fish would cost Redhook $10 million annually) and we need go choose conclusion not the same or paraphrased statement.

Editorial: Almost every year the Smithfield River floods the coastal fishing community of Redhook, which annually spends $3 million on the cleanup. Some residents have proposed damming the river, which would cost $5 million but would prevent the flooding. However, their position is misguided. A dam would prevent nutrients in the river from flowing into the ocean. Fish that now feed on those nutrients would start feeding elsewhere. The loss of these fish would cost Redhook $10 million annually.
Summary: build dam is suggested but it will be a mistake as it will do more damage

Which one of the following most accurately expresses the main conclusion of the editorial’s argument?

(A) The Smithfield River should be dammed to prevent flooding.
(B) Nutrients from the Smithfield River are essential to the local fish population.
(C) Damming the Smithfield River is not worth the high construction costs for such a project.
(D) For Redhook to build a dam on the Smithfield River would be a mistake.
correct
(E) The Smithfield River floods cost Redhook $3 million every year.

Editorial: Almost every year the Smithfield River floods the coastal fishing community of Redhook, which annually spends $3 million on the cleanup. Some residents have proposed damming the river, which would cost $5 million but would prevent the flooding. However, their position is misguided. A dam would prevent nutrients in the river from flowing into the ocean. Fish that now feed on those nutrients would start feeding elsewhere. The loss of these fish would cost Redhook $10 million annually.

Which one of the following most accurately expresses the main conclusion of the editorial’s argument?

(A) The Smithfield River should be dammed to prevent flooding. - WRONG. Its like C only.
(B) Nutrients from the Smithfield River are essential to the local fish population. - WRONG. Can be an assumption but not conclusion.
(C) Damming the Smithfield River is not worth the high construction costs for such a project. - WRONG. Construction of dam is at all not promoted in the passage. Checking its worthiness is therefore not at all concerned.
(D) For Redhook to build a dam on the Smithfield River would be a mistake. - CORRECT. Because of this the highlighted text makes sense as in why editorial said so.
(E) The Smithfield River floods cost Redhook $3 million every year. - WRONG. Can't be a conclusion as already mentioned. It can be used to further the conclusion.

Answer D.