E. coli has been isolated and cultured from three different individuals. Upon biochemical testing of these three cultures, you find that there are differences in some reactions. What is the best explanation of these differences?

A) The other two cultures are not really E. coli.
B) Two of the cultures are mutants of the other.
C) Mistakes have been made in the test interpretation.
D) This is normal genetic variation seen among strains of E. coli.
E) These represent three different species of E. coli.