The bacterium E. coli can use carbon sources other than glucose for nourishment. Arabinose, a five-carbon sugar, is one example of such an
alternative food source. However, E. coli does not metabolize arabinose if
glucose is plentiful in its environment. E. coli only metabolizes arabinose if it
is the only food source available. Which of the following best explains how
the cell's function is enhanced by this gene structure?

A. This sequence of genes ensures that the enzymes needed for
arabinose metabolism are transcribed in the same order in which
the cell will need them.

B. The regulator, activator, and promoter regions ensure that the
genes for metabolizing arabinose are not mutated or deleted
during DNA synthesis.

C. The regulator gene ensures that the genes are transcribed at all
times, to be sure the cell is ready in case arabinose becomes
available.

D. This gene sequence ensures transcription of arabinose genes only
when arabinose is available, saving the cell from expending energy
unnecessarily

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