Question

read the paragraph, then choose the word that best completes the sentence.
there is a zoo of critters living on the surface of ones skin—bacterial critters, that is. scientists have long known that healthy skin hosts bacteria, but a recent study revealed that this miniscule community is far larger and more diverse than first realized. invisible to the naked eye, trillions of bacteria, fungi, and other microbes naturally coexist. these organisms feed on dead skin cells and the secretions on the skin produced from glands. this is not a bad thing. according to genetics specialist julia segre, most bacteria are helpful. they keep the skin moisturized and wounds safe from other, more harmful, bacteria.
the benefits to organisms and humans are ______.
a erratic
b synthetic
c mutual
d transient

Explanation:

The paragraph states that microbes feed on dead skin cells and skin secretions (benefiting themselves) while also keeping the skin moisturized and safe (benefiting humans). The word "mutual" means giving benefits to each other, which fits this relationship. "Erratic" means inconsistent, "synthetic" means artificial, and "transient" means temporary, none of which fit the context of a mutually beneficial relationship.

Answer:

C. mutual