There is a small phrase right in the middle of the selection that says <em>impact the fragile environment. </em>
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The language suggests that there is some concern about what microbes can do. Some are very necessary, but they can mutate and become dangerous. The answer is B, but don't be surprised if it isn't.
The quotation is a bit disjoint. You think that it is going to be about water by the lead sentence. It isn't. A is incorrect.
There is no mention of the microbes disappearing. Quite the opposite. They are part of every ecosystem.
The paragraph makes it quite clear that we can measure the way microbes change and are quite observable as a consequence. D is not correct.\\
