Standing is not allowed in a double decker bus. Why ?

Passengers are to travel by a double decker bus which can accomodate in upper deck and in lower deck.The number of ways that the passengers are distributed is

Why are passengers travelling in a double decker bus allowed to stand in a lower deck, but not in the upper deck?

There are 10 seats in a double decker bus,6 in the lower deck and 4 on the upper deck.Ten passengers board the bus,of them 3refuse to go to the upper deck and 2 insist on going up. The number of ways in which the passengers can be accommodated is (Assume all seats to be duly numbered)

Which of the following are concave mirrors and which convex mirror ? Shaving mirrors, Car headlight mirror, Searchlight mirror, Driving mirror, Dentist's inspection mirror, Torch mirror, Staircase mirror in a double-decker bus, Make-up mirror, Solar furnace mirror, Statellite TV dish, Shop security mirror.

Five different coloured buses are standing in a row facing South. Black coloured bus is standing in the immediate right of Red. Green colour is between Blue and Yellow. Yellow colour is between Black and Green. Which coloured bus is standing in the middle ?

A capacitor allow a.c. to pass through. Why ?