An engine is hauling a train of mass M kg on a level track at a constant speed v m/s. The resistance due to friction is\[fN/kg\]. What extra power must the engine develop to maintain the speed up a gradient of h in s :

• A. \[\frac{mghv}{s}\]
• B. \[\frac{mghs}{v}\]
• C. \[Mghvs\]
• D. zero