What is the determining factor for the transcription of structural genes of the trp operon?

A.
the quantity of trp repressor protein made by the regulatory gene trpR
B.
the ability of RNA polymerase to bind to the promoter region
C.
the availability of tryptophan in the environment
D.
the location of a regulatory gene with reference to the other regions of the operon
E.
the activity of the other operons present in bacteria