Excluding stoppages, the speed of a bus is 40 kmph and including stoppages, it is 30 kmph. For how many minutes does the bus stop per hour? Choose the correct choice from the following:
A) 10 min B) 12 min C) 18 min D) 15 min E) 20 min

Rationale: "due to stoppages , it covers 10 km less . time taken to cover 10 km = ( 10 / 40 x 60 ) min = 15 min answer : d"
Answer: D