The Ames test is commonly used to test whether a particular chemical is mutagenic. When the Ames test is conducted, two different groups of plates of culture medium in which histidine is omitted are inoculated. One group of plates is inoculated with bacteria that are exposed to the chemical of interest, while the other plates are inoculated with bacteria that are not exposed to the chemical. Why is the latter group needed if you are just interested in the rate of mutation caused by the chemical?

a) Two groups are not needed. It is only necessary to count the number of colonies that form on the plate that has the chemical of interest.
b) The second group is used as a control to make sure that the replica plating technique is done correctly.
c) The group without the chemical can be exposed to mammalian liver extract to see the effects of liver enzymes on the acteria as an additional treatment.
d) It is important to compare the rate of mutations with the chemical to the normal background mutation rate. Mutations occur spontaneously even without the presence of a mutagen.