(DDT) is a chemical that was used widely to control pests in agricultural crops. DDT was banned in many countries in the 1970's when many predatory birds were experiencing thinning of their egg shells. The thinning led to low birth rates and low infant survival, which threatened some populations. The birds' habitat was too far from the crops for the birds to have been exposed directly to the DDT. The most likely explanation for DDT exposure in these birds, despite their distance from the source is

Select one:
a. the birds sometimes used the treated grain in building their nests, with the DDT leaching into the egg shells
b. that DDT remained in the tissue of plants, which were eaten by primary consumers and then the birds (at each tropic level, the concentration of DDT increased and caused the most harm to predatory birds)
c. the DDT remained in the air and was carried to the bird nests by wind and rain
d. that some other toxic chemical was harming the egg shells