California Condors, Large, Predatory Birds That Were Previously on the Edge

Multiple Choice

California condors, large, predatory birds that were previously on the edge of extinction, have benefited from a program that raises birds in captivity and then releases them into the wild. The reintroduction program, however, is having variable success.
-Scientists suspected that the condors eating marine mammals were ingesting DDT, causing the kind of shell fragility found in the middle part of the last century before DDT was banned. Which action would provide the best evidence that poor egg quality rather than predation or some other factor was the cause of failed condor reproduction?

A) swapping eggs from nests of reintroduced condors with eggs from successfully reproducing condors in captivity
B) comparing the number of eggs in nests of birds in captivity with the number of eggs in the nests of birds in the wild
C) counting the number of mating pairs in each area in the wild
D) monitoring food eaten by condors in each of the areas where condors have been reintroduced