In paragraph 6, the author compares the energy costs of vigorous begging with the potential gain in calories from such begging in order to A. explain why begging for food vigorously can lead to faster growth and increased size B. explain how begging vigorously can increase an individual's chance of propagating its own genes C. point out a weakness in a possible explanation for why nestlings do not always beg vigorously D. argue that the benefits of vigorous begging outweigh any possible disadvantages