Starlings Sometimes Assemble in Large Flocks That, in the Evening

Starlings sometimes assemble in large flocks that, in the evening, rise up in whirling flight together for 15 or 20 minutes in what looks like some kind of display. V.C. Wynne-Edwards suggested that this behavior is indeed a display that enables individual birds to assess just how many fellow starlings are living in the area so that they can adjust their breeding effort in the upcoming breeding season. In this way, they can avoid producing too many chicks and thereby avoid overpopulating their home range. This a group selectionist hypothesis because

A) the idea is that starlings could evaluate the size of their local population or group.
B) the flight behavior of the flock is something that cannot be explained in terms of any benefits to individuals and, thus, must be beneficial for the group.
C) starlings do breed in such a manner as to avoid overpopulation.
D) the suggestion is that the birds engage in costly behaviors in order to make decisions that are advantageous for other unrelated starlings.