If both the pipes are opened, how many hours will be taken to fill the tank

I. 
The capacity of the tank is 400 litres

II. 
The pipe A fills the tank in 4 hours
 
III.
 
The pipe B fills the tank in 6 hours

Only I and II

Only II and III

All I, II and III

Any two of the three

Even with all the three statements, answer cannot be given.