A resistor obeys the relationship V=IR, where V is voltage, I is current, and R is resistance. At one instant the voltage across the resistor is decreasing while the current through it is increasing. Which of the following best describes the instantaneous rate of change of the resistance, dR/dt, at that moment?
a) dR/dt is positive.
b) dR/dt is zero.
c) dR/dt is negative.
d) The sign of dR/dt cannot be determined from the given information.