How Can Normal Flora Be a Barrier to Entry

Multiple Choice

How can normal flora be a barrier to entry?

A) They cause an inflammatory response that kills pathogenic bacteria upon contact.
B) They phagocytize any pathogenic bacteria that attempt to infect.
C) They use up nutrients and release wastes, preventing pathogenic bacteria from gaining a foothold.
D) They release histamine, which causes blood vessels to become permeable.
E) They activate the complement system, which punches holes in the invading pathogen.