Which of the following are plausible hypotheses that could explain why mammals never evolved unidirectional airflow through their lungs, while birds did? Select one or more: a. Mammals may have never experienced a need for unidirectional flow in their evolutionary past. b. The necessary alleles of developmental genes to develop associated structures of unidirectional flow never arose in mammals. c. Mammal-like lungs and bird-like lungs occupy different fitness optima in terms of having optimized capacity for gas exchange, and individuals of intermediate phenotype necessary to change to a bird-like lung would have to cross a valley in the fitness landscape. d. Unidirectional airflow would only benefit individuals but not the species overall. e. The necessary changes might need to happen at such an early developmental stage that altering the expression patterns of the genes involved might cause some additional severe developmental anomalies.