The diagram illustrates a lift arrangement where the elevator (mass $m_1$) is partly balanced by a large suspended mass (mass $m_2$). At what rate is energy supplied to the system by the motor when the elevator is moving upwards at a constant speed $v$? ($g$ = acceleration of free fall)
- A$\frac{1}{2} m_1 v^2$
- B$\frac{1}{2} (m_1 - m_2) v^2$
- C$m_1 g v$
- D$(m_1 - m_2) g v$