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: how many paying stones , each measuring 2 m * 2 m are required to pave a rectangular court yard 30 m long and 18 m board ? 
Options: a ) 99 , b ) 18 , c ) 135 , d ) 17 , e ) 12
Example Output: c

Example Input: Problem: a searchlight on top of the watchtower makes 4 revolutions per minute . what is the probability that a man appearing near the tower will stay in the dark for at least 12 seconds ? 
Options: a ) 1 / 2 , b ) 2 / 3 , c ) 1 / 3 , d ) 1 / 4 , e ) 1 / 5
Example Output: e

Example Input: Problem: a and b complete a work in 9 days . a alone can do it in 18 days . if both together can do the work in how many days ? 
Options: a ) 6 days , b ) 8 days , c ) 5 days , d ) 7 days , e ) 15 days
Example Output:
a