The reason why Beadle and Tatum observed four different categories of mutants that could not grow on media without methionine is because

a. the enzyme involved in methionine biosynthesis is composed of four different subunits.

b. the enzyme involved in methionine biosynthesis is present in four copies in the Neurospora genome.

c. four different enzymes are involved in a pathway for methionine biosynthesis.

d. a lack of methionine biosynthesis can inhibit Neurospora growth in four different ways.