Situation: The passage describes two kinds of results of water contamination tests: false positives and false negatives. It then recommends using the lowest false-positive test to get the most accurate results.
Reasoning: There are two types of false results; the author recommends maximizing accuracy by focusing only on false positives. The author is therefore assuming that false negatives don't matter, so the correct supporting statement will reinforce this belief.
Make sure to stay focused on the author's exact conclusion. Choices (A) and (C) both relate to the consequences of getting a false positive or a false negative result. This makes them tempting traps, but off topic--we are concerned with accurate results only, not harmful or harmless results. Similarly, choices (B) and (D) are off-topic, since we're concerned only about accurate results for mercury, not about inconclusive mercury tests or tests for other metals.
Choice (E) matches our prediction perfectly. It states that all tests have the same false negative frequency, which supports the author's conclusion that false positives are the primary determinant of accuracy.
As a result, Choice (E) is certainly correct.