Two people, each homozygous ro autosomal recessive deafness-producing alleles have four children. Three are deaf. One can hear. Possible explanations for the appearance of the hearing child include all of the following EXCEPT
a - complementation of two different loci (AA bb x aa BB --> Aa Bb [hearing])
b - back mutation (dd --> Dd)
c - heterozygosity at loci for which spouse is homozygous (Aa bb x aa Bb --> Aa Bb)
d - infidelity / mistaken parentage
e - late or variable age of onset (hearing child will lose hearing later in childhood)