This is very comprehensive, but I think that your questions will be answered, so please bear with me.
A. Studies have shown that besides bacteria, many other kinds of pathogens can be found in regularly used washing-machines.Within scope - "any health risks" is the conclusion drawn by the author. Such a generalization would find weakness by introducing health risks other than that caused by bacteria. Do we know that bleach kills these other pathogens? Since we must take this answer choice as true, it brings considerable doubt and/or questions to whether bleach can eliminate any health risks, and the author's conclusion is hasty and insufficiently supported.
This is the answer.
B. Undiluted bleach is not more powerful than diluted bleach in killing bacteria.This may sound as if it weakens, but it's irrelevant. It may require the author to modify the conclusion to say that either diluted or undiluted bleach will be effective in killing bacteria... in fact, I argue that the author doesn't even have to modify the conclusion, as the author only makes a claim in the conclusion with respect to diluted bleach (yes, the argument brings up details about undiluted bleach causing damage to washing machine drums, but the conclusion is about
how extreme that info is interpreted - e.g. restate the conclusion "only with respect to diluted bleach, we can conclude X").
Anyway, the point still remains that undiluted bleach will still cause damage to the machine. Thus, you'd still want to use diluted bleach instead of undiluted bleach, as the author suggests.
C. Many companies come with gentle bleach blends that do not corrode the metal drum of the household washing machines.This actually
strengthens the author's argument. It's similar to restating the info that's already mentioned, as gentle bleach
blends is a type of diluted bleach. It's
not irrelevant - if undiluted bleach seriously damages washing machine drums, then diluted bleach's effects on washing machine drums cannot be inferred. So, the author makes a leap in concluding that diluted bleach is safe for machines.
Also, as a very minor aside, if diluted bleach wasn't available (e.g. "there are no bleach blends..."), it would be equivalent to suggesting a magical solution, as if-then logic dictates that any false hypothetical can be followed by a true or false conclusion, making the entire logical equation true (e.g. if it rains chocolate outside everyday, then I am a billionaire = true statement)! Then it seriously undermines the author's conclusion because blends or diluted bleach may not be available for purchase for use in washing machines, making the author's solution ineffective and entirely irrelevant. Thus, with the wording changed, choice C would be
the answer, as bleach cannot be used to kill any bacteria, and will knock out the actual answer to this question, which at least kills some bacteria even if more pathogens are introduced (unavailable bleach effectively eliminates no health risks vs., bleach eliminates some or possibly even all health risks, respectively).
D. Various studies have shown that certain bacteria can live outside the human body for long periods of time on various objects such as clothes, toys etc.This only adds to the given info with very little value-added. This answer choice reinforces the background detail to generate more support for the author's conclusion.
Conclusion: Use diluted bleach...
Author's support: because washing machines transfer bacteria onto clothes, which...
Answer Choice D: "Various studies have shown that certain bacteria can live outside the human body for long periods of time on various objects such as clothes, toys etc."...
E. Most household washing machines come with a hot-water cycle that is set at a temperature of 90 degrees Celsius, a temperature that has been cited as ideal for killing various disease-causing bugs in these machines.This may sound as if bleach is unnecessary, but the point still remains... if the hot-water cycle is sufficient at killing various disease-causing bugs, which is indeed a health risk, then are other disease causing sources eliminated? It's confined to just disease-causing bugs and does not necessarily pertain to bacteria, or any other health risks for that matter. In fact, it can be used to support the conclusion, in that the hot-water cycle can combine with the diluted bleach to eliminate (more, not necessary any and all) health risks. Does it weaken or support the author's conclusion? Clearly, this answer choice has commitment issues.