Task: Question Answering
Input (English): Problem: a leak in the bottom of a tank can empty the full tank in 9 hours . an inlet pipe fills water at the rate of 6 litres a minute . when the tank is full , the inlet is opened and due to the leak , the tank is empty in 12 hours . how many litres does the cistern hold ? 
Options: a ) 7580 , b ) 12960 , c ) 8290 , d ) 8640 , e ) none
Output (English): b