Why Might the Second, Unregulated, Tryptophan Promoter Site in the Trp

Why might the second, unregulated, tryptophan promoter site in the trp operon be needed?

A) In case there is a mutation in the other promoter that prevents expression
B) To allow for synthesis of the trp repressor protein
C) To attenuate transcription when trp is present
D) To provide a constant low level of necessary trp-containing enzymes
E) To provide dyad symmetry of the operator