Is this an excercise in a class ?
I ask because it is a very unusual combination, and there are problems with the safety:
No power to the STOP button: Device will continue to run and cannot be stopped.
No power to the START button: Device will start immediately !
Sounds like your teacher said something like:
"NO for start and NC for stop is the normal thing, what would happen if the reverse was the case ?"
You should answer something like "That would be very dangerous, Sir.".
It is quite common for the STOP button to be NC because it increases the safety in most cases.