In the Case of the Schallers,why Doesn't Their Genotype Correlate

In the case of the Schallers,why doesn't their genotype correlate with their phenotype?

A) Although each carries a copy of a mutated CF gene,they also express a normal copy of the CF gene that renders them phenotypically normal.
B) The Schallers have only normal,nonmutated copies of the CF gene.
C) The Schallers inherited one mutated and one normal CF gene.
D) None of the above.
E) A and C.