A hydroelectric power station converts the gravitational potential energy of water into electrical energy. In one specific power station, the mass of water passing per unit time is $1.5 \times 10^5\ \text{kg s}^{-1}$. The water descends through a vertical drop of $120\ \text{m}$. The electrical power output is $100\ \text{MW}$. Calculate the efficiency of the power station?
- A5.6%
- B43%
- C57%
- D77%