If You Live on a River Delta or Coastline,you May

If you live on a river delta or coastline,you may not be able to eat the local shellfish because of high levels of toxins,even though you may be able to drink the water taken from the same source.Why?

A) Shellfish seasonally produce toxic substances.
B) Shellfish regularly consume poisonous red tide organisms.
C) Most harmless native clams have been replaced by bad species.
D) Shellfish filter-feed and concentrate the dilute pollutants from the water,thus acting as early warning indicators of severe pollution.