Which of the following is true about penalty, where D(t) is instantaneous demand at time t and R(t) is response at time t?

a) Penalty ∝ ∫ | R(t) * D(t) | dt
b) Penalty ∝ ∫ | D(t) + R(t) | dt
c) Penalty ∝ ∫ | D(t) – R(t) | dt
d) None of these