The Binding Affinity of ACh for Its Fetal Receptors at the Site

The binding affinity of ACh for its fetal receptors at the site of innervation vs. that at the end-plate in adult muscle is likely because of the

A) sheer number of receptors that can occupy the respective sites.
B) different subunit composition of between the fetal and adult forms of the receptor.
C) density of the receptors at the site of innervation between the fetal and adult forms of the receptor.
D) size of the presynaptic terminal.
E) differences in surface area between the fetal and adult sites of innervation.