单选题 .  ①What causes size variation in bumblebee workers? ②In pollen storing species larvae develop in individual nest cells and are fed directly by adults. ③Thus, adults probably determine the size attained by each larva. ④Bumblebee larval cells are not as neatly organized as are those of honeybees, and larvae at the periphery of the brood may receive less care than others. ⑤Some researchers may thus believe that ten-fold variation in worker mass results from the accidental neglect of some larvae. ⑥However, given that larvae are reared in a controlled environment by specialized nest workers, it seems more likely that this size variation has an adaptive function and that the colonies benefit from rearing workers of a range of sizes.5.  It can be inferred that the author of the passage considers the variation in size of bumblebee workers to be ______