Guitar strings often go "dead"--become less responsive and bright in

The researcher hypothesizes that when guitar strings go "dead" after a few weeks of intense use, dirt and oil, rather than changes in the material properties of the string, are responsible. This hypothesis does not exclude the possibility that other factors would cause the strings to go dead at different rates.

For example, say we have 10 guitars from 10 different brands all used intensely for a few weeks, and the strings on those guitars go dead at slightly different rates. In that case, it is still possible that dirt and oil were primarily responsible and that other brand factors slightly influenced the rates (or maybe the design of some brands is simply more susceptible to dirt and oil). This result would not suggest that those other factors, in the absence of dirt and oil, would cause the strings to go dead after a few weeks of intense use.

True, such an experiment could yield information that might weaken the conclusion, but we are looking for the investigation that is the "most likely to yield significant information that would help to evaluate the researcher's hypothesis." Because the experiment described in choice E specifically investigates dirt and oil, the results are likely to help us evaluate the researcher's hypothesis, and so choice E is much better than choice C.

I hope this helps!