光滑的水平面叠放有质量分别为m和m/2的两木块，下方木块与一劲度系数为k的弹簧相连，弹簧的另一端固定在墙上，如图所示．已知两木块之间的最大静摩擦力为f，为使这两个木块组成的系统象一个整体一样地振动，系统的最大振幅为（　　）

A．

B．

C．

D．

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 to evaluate the researcher’s hypothesis

A．Determining if a metal alloy is used to make the strings used by classical guitarists.

B．Detennining whether classical guitarists make their strings go dead faster than do folk guitarists.

C．Detennining whether identical lengths of string, of the same gauge, go dead at different rates when strung on various brands of guitars.

D．Determining whether a dead string and a new string produce different qualities of sound.

E．(E) Determining whether smearing various substances on new guitar strings causes them to go dead.