A Student Complains That It Makes No Sense to Worry

A student complains that it makes no sense to worry about coliform bacteria in water,since we naturally possess harmless coliforms in our intestines anyway.Why do regulatory agencies worry about coliform bacteria in water supplies,then?

A) The coliforms in our intestines can sometimes turn harmful,causing intestinal infections.We need to constantly be on guard against this happening.Coliforms in water might add to this potential problem.
B) Not all coliforms are harmless and symbiotic with human beings.Some may carry genes/proteins that can make them dangerous to humans.It's best to keep coliforms OUT of our drinking water,since it's difficult to identify which ones might be harmless and which ones might be harmful.
C) Regulatory agencies are worried that multiple types of coliforms in water might lead to greater antibiotic resistance in the bacterial populations.They try to keep them out of water to keep antibiotic resistance levels down.
D) Regulatory agencies simply need something to do to justify their existence.There is no real danger from coliforms in water supplies.