Which of the following best explains why E. coli shuts down the trp operon if tryptophan is available in the environment?

A) It does not shut down the trp operon, it only lowers the level of trp operon activity.
B) The trp operon encodes genes that export tryptophan from the cell; if tryptophan already occurs in the environment, further export is not necessary.
C) Synthesizing an amino acid takes energy, so it is a waste of energy to make something that is already available.
D) Environmental tryptophan is of higher quality than what the E. coli can make itself.