单选题 .  ①Biologists have long debated about whether egg production in birds is biologically highly costly, some theorizing that egg production is energetically or nutritionally demanding. ②Lack, however, suggested that clutch size—the number of eggs a bird lays per breeding cycle—is far below the potential limit of egg production. ③He suggested that clutch size had instead evolved in relation to the number of young that the parents could successfully rear. ④Subsequently, most studies focused on limitations operating during chick rearing, particularly among altricial species (species in which the parents feed their young in the nest). ⑤Lack later recognized that in precocial species (species in which young feed themselves), clutch size might be explained by different factors—the availability of food for egg-laying females, for example.48.  The passage suggests that biologists who say egg production in birds is biologically highly costly would agree that clutch size is determined primarily by ______