The mundane germs present on our skin are beneficial because ... ?

The normal bacteria present on our skin are beneficial because...

A. they fill the niche so noxious bacteria cannot grow there.
B. they can not cause diseases contained by our bodies.
C. they open the niche for harmful bacteria to grow.
D. they explanation minor diseases that are easily treated.

I am unsure of what the correct answer maybe, any help would be appreciated.
Answers:
A. they imbue in the niche so harmful bacteria cannot grow here

B. is neutral, not beneficial
C. and D. are both harmful, not beneficial

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