Biofilms are difficult to inactivate in water distribution systems because:
a. The pipes corrode under low oxygen conditions and aid in the metabolism of biofilm microbes helping them survive
b. There are too many different types of microbes associated to biofilms making it hard to use the right chemical agent
c. The glycocalyx that binds microbes in biofilm together also protects them from chemical disinfectants
d. Most have developed antibiotic resistance