A discusses the process of dilution rather than the process of cleaning the machine. So eliminate A.

B says diluted has to be "at least" as effective as concentrated. This is not necessarily true. So I am going to eliminate B.

C discusses concentrated bleach rather than diluted. So eliminate C.

D discusses another factor that needs to be taken into account for the elimination of health risks conclusion to hold. D could be right.

E discusses cost rather than safety and efficacy. So eliminate E.

Choose D.

Although the question stem is a little different, this is an assumption question. On the actual GMAT nearly every assumption question will say "assumption" or "assumes."

In this case "The argument above will not hold unless which of the following statements holds true?" Is asking for the assumption.

Focus on the conclusion. Assumption questions, like strengthen and weaken questions, are best approached by first identifying the conclusion.

In this case the conclusion is, "Thus, bleach, as long as it is diluted properly, is an effective and safe means of eliminating any health risks from such washing machines." For this conclusion, the portion that I have highlighted in bold is what I like to call something that the argument "Did not have to say." This can point you in the right direction in terms of analyzing the argument.

They did not have to say that bleach eliminated "ANY HEALTH RISKS" the argument was in fact much more directed to a conclusion that they limit the health risks FROM THE BACTERIA.

If you noticed that they argument suddenly broadened the scope from bacteria in the evidence to "any health risk" in the conclusion then you can anticipate that the answer choice might actually bring in another health risk.

That is what choice D does. While the test taker is wondering if the diluted bleach is still strong enough to deal with the bacteria - and probably choosing choice B. The correct answer is dealing with the possibility of harmful residues from the bleach itself.

So choice D is the correct answer because without it their could be health risks, specifically from the diluted bleach itself.

D. Diluted bleach does not leave any harmful residues in the washing machine that attach themselves to our clothes.

It is ineffective when mixed with organic matter..Also,Bleach is as much of a toxin as it is a cleaner.