Adult male humans generally have deeper voices than do adult female humans, which is the direct result of higher levels of testosterone causing growth of the larynx. If the fossil records of apes and humans alike show a trend toward decreasing larynx size in adult females and increasing larynx size in adult males, then .a. sexual dimorphism was developing over time in these speciesb. intrasexual selection seems to have occurredc. stabilizing selection was occurring in these species concerning larynx sized. selection was acting more directly upon genotype than upon phenotype