Normal flora are helpful when they are .
a. within tissues
b. on body surfaces
c. in the blood
d. excreted
e. able to combat MRSA

Normal flora are helpful when they are within tissues

Explanation:

Normal flora are organisms that are naturally found especially within tissues they include bacteria which are natural flora of such habitat

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explanation:

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genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).