In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Example Input: Problem: a man cycling along the road noticed that every 18 minutes a bus overtakes him and every 6 minutes he meets an oncoming bus . if all buses and the cyclist move at a constant speed , what is the time interval between consecutive buses ?
Options: a ) 5 minutes , b ) 6 minutes , c ) 8 minutes , d ) 9 minutes , e ) 10 minutes
Example Output: d

Example Input: Problem: a cistern of capacity 8000 litres measures externally 3.3 m by 2.6 m by 1.2 m and its walls are 5 cm thick . the thickness of the bottom is :
Options: a ) 90 cm , b ) 2 dm , c ) 1 m , d ) 1.1 cm , e ) none of these
Example Output: b

Example Input: Problem: a train running at the speed of 60 km / hr crosses a pole in 4 seconds . find the length of the train .
Options: a ) 66.67 , b ) 882 , c ) 772 , d ) 252 , e ) 121
Example Output:
a