Which of the following situations might cause normal microbiota to become opportunistic pathogens?

A) growth of microbes on the surface of intact skin
B) growth of microorganisms on the excreted cellular wastes and dead cells in the large intestine
C) treatment of a cancer patient with radiation
D) growth of Lactobacillus on the surface of teeth
E) presence of Entamoeba in the lumen of the colon