The fluctuation test of Luria and Delbruck showed that:

A)a selecting agent can affect mutation rate in E.coli.
B)mutations can arise spontaneously before exposure to the selecting agent.
C)mutations in E.coli occur at a relatively high frequency.
D)the mutation rate in E.coli fluctuates greatly from one generation to the next.
E)the T1 phage can act as a mutagen as well as a selecting agent in E.coli.