Which of the following statements supports the one gene-one enzyme hypothesis?

a) a mutation in a single gene can result in a defective protein.
b) sickle-cell anemia results in normal hemoglobin.
c) multiple antibody genes can code for different related proteins, depending on the splicing that takes place post-transcriptionally.
d) alkaptonuria results when individuals lack multiple enzymes involved in the catalysis of homogentisic acid.