Beadle and Tatum discovered several classes of Neurospora mutantsthat were able to grow on minimal medium with arginine added. Class I mutants were also ableto grow on medium (no arginine) supplemented with either ornithine or citrulline, whereasclass II mutants could grow on citrulline medium but not on ornithine medium. The metabolicpathway of arginine synthesis is as follows:

Precursor —> Ornithine —-> Citrulline —–>Arginine
                A                   B                     C
From these growth results, they could conclude that

a. One gene codes for the entire metabolic pathway
b. The genetic code of DNA is a triplet code
c. Class I mutants have their mutations later in the nucleotidechain than do class II mutants, and thus have more functional enzymes
d. Class I mutants have a nonfunctional enzyme at step A, and classII mutants have a nonfunctional enzyme at step B
e. Class I mutants have a nonfunctional enzyme at step B, and classII mutants have a nonfunctional enzyme at step C