Which of the following best explains why herring gull chicks were one of the first species to exhibit the effects of pollution, according to the figure? a. toxins increase in concentration as one moves up a food chain. b. toxins have less of an effect on herring gulls than on other species. c. toxins affect terrestrial organisms more severely than aquatic organisms. d. toxins decrease in concentration as one moves up a food chain.

The correct answer is A.
Toxins and various pollutants increase in concentration through the food chain in a process called biomagnification. This means that the organisms on the top of the food chain, like the herring gull, have many times greater concentration of toxins than the lower food chain level organisms because they prey on lower food chain level organisms. If a herring gull ears 100 herrings, it will have a 100x times greater concentration of toxins in its body than a herring.