A manufacturer of cases for sound equipment requires that holes be drilled for metal screws. The drill bits wear out and must be replaced; there is expense not onlyr in the cost of the bits but also in the cost of lost production. Engineers varied the rotation speed of the drill and measure the lifetime of four hits at each of ve speeds. In this regession model, which is the explanatory variable? (:3 lifetime of drill bits speed at which holes were drilled lifetime of drill bits and speed at which holes were drilled None of the listed answers is correct