Guitar strings often go "dead"-become less responsive and bright in tone-after a few weeks of intense use. A researcher whose son is a classical guitarist hypothesized that dirt and oil, rather than changes in the material properties of the string, were responsible. Which of the following investigations is most likely to yield significant information that would help evaluate the researcher's hypothesis? ['Determining whether a metal alloy is used to make the strings used by classical guitarists', 'Determining whether classical guitarists make their strings go dead faster than do folk guitarists', 'Determining whether smearing various substances on new guitar strings causes them to go dead', 'Determining whether a dead string and a new string produce different qualities of sound']