When we're asked what information would help us to EVALUATE an argument, we're looking for a question for which a "yes" answer would weaken the argument and a "no" answer would strengthen the argument (or vice versa).

Premise: Guitar strings often go "dead"--become less responsive and bright in tone--after a few weeks of intense use.

Conclusion: dirt and oil, rather than changes in the material properties of the string, are responsible.

Missing information:
- do we know that dirt and oil accumulate in this amount of time? What happens in sanitary conditions when there is no dirt & oil accumulation?
- how do we know that the material properties of the string did not change?

With each of the answer choices, ask yourself: what would a "yes" answer do to the conclusion? What would a "no" answer do?

(A) Determining if a metal alloy is used to make the strings used by classical guitarists
- if yes... strings could still go dead from dirt or from material changes.
- if no... same thing.

(B) Determining whether classical guitarists make their strings go dead faster than do folk guitarists
- nothing in the argument suggests that classical v. folk guitarists have different oil & dirt patterns. Irrelevant.

(C) Determining whether identical lengths of string, of the same gauge, go dead at different rates when strung on various brands of guitars
- if yes... it could still be oil & dirt or it could be material changes.
- if no... same thing.

(D) Determining whether a dead string and a new string produce different qualities of sound
- we're already told that "dead" by definition is less bright in tone. This is something we already know, not something that would be useful to know.

(E) Determining whether smearing various substances on new guitar strings causes them to go dead .
- if yes... this would confirm the dirt & oil theory.
- if no... this would disprove the dirt & oil theory.
This is the thing that would be helpful to know!

The answer is E.