Scenario Drs. Peter and Rosemary Grant Have Been Conducting Observational Research

Scenario
Drs. Peter and Rosemary Grant have been conducting observational research studies on finch species on Galápagos Island Daphne Major for over 40 years. The medium ground finch has a relatively small beak and a diet that consists primarily of small seeds. Large-beaked finches are able to eat larger seeds in addition to small ones. These two species successfully co-existed on the island for over 20 years, but two years of severe drought in 2003 and 2004 lead to intense competition between them because food supplies were destroyed in the drought. Numerous birds in both populations died from starvation. Interestingly, the Grants noticed that the smaller-beaked finches did not face the same degree of competitive pressure, because they could more easily eat the smaller seeds that were not being eaten by the larger finches. Only a short time later (2004-2005) , the Grants observed that new populations of finches had more birds with smaller beaks than larger ones. As well, finches tended to seek out larger seeds half as often as in preceding years.
-In these observational studies, what would have served as a control experiment?

A) A population of finches with large beaks that faced excessive rainfall and high winds and were not able to feed normally.
B) A population of finches with small beaks that faced natural fires on the island and were not able to feed normally.
C) Populations of finches with small and large beaks that did not face drought and were able to feed normally.
D) Populations of a bird species genetically unrelated to finches that did not face drought and were able to feed normally.