How Does Dental Unit Water Become So Highly Contaminated

How does dental unit water become so highly contaminated?

A) The incoming city or well water becomes highly contaminated by the time it reaches the office, so this contamination passes through the dental unit.
B) Contaminated room air enters through the tip of the air/water syringe and the bur hole of high-speed handpieces contaminating the water spray as it exits the waterline.
C) Biofilm builds up in the waterlines and contaminates the water as it flows through the line.
D) All of the microbes in dental unit water come from patients' saliva that has been retracted back into the line through the high-speed handpieces.