C
Feathers recently taken from seabirds stuffed and preserved in the 1880s have been found to contain only half as much mercury as feathers recently taken from living birds of the same species. Since mercury that accumulates in a seabird' s feathers as the feathers grow is derived from fish eaten by the bird, these results indicate that mercury levels in saltwater fish are higher now than they were 100 years ago. The argument depends on assuming that
A: mercury derived from fish is essential for the normal growth of a seabird's feathers
B: the proportion of a seabird's diet consisting of fish was not as high, on average, in the 1880s as it is today
C: the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds' feathers
D: the amount of mercury in a saltwater fish depends on the amount of pollution in the ocean habitat of the fish