A battery is charged at a potential of 15 V for 8 hours when the current flowing is 10 A. The battery on discharge supplies a current of 5 A for 15 hours. The mean terminal voltage during discharge is 14 V. The "Watt hour" efficiency of the battery is :

• A

  80%

• B

  90%

• C

  87.5%

• D

  82.5%