
David H. answered 02/19/19
MS2 student for Math and Science Tutoring
Simmon's citrate agar is used for the differentiation of gram negative bacteria on the basis of citrate utilization with bacteria that contain this property as their sole carbon source turn the medium blue due to the incorporation of bromthylmol blue.
A false positive can occur if a heavy inoculum is used instead of a light quantity, with the preformed organic compounds within the cell walls of dying bacteria may release sufficient carbon and nitrogen to produce a false positive test result.
So overall the answer to your question will be none of the above.