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 and b together can do a piece of work in 6 days and a alone can do it in 13 days . in how many days can b alone can do it ? 
Options: a ) 12 days , b ) 15 days , c ) 78 / 7 days , d ) 21 days , e ) 22 / 7 days
Example Output: c

Example Input: Problem: if the wheel is 21 cm then the number of revolutions to cover a distance of 1056 cm is ? 
Options: a ) 8 , b ) 27 , c ) 10 , d ) 7 , e ) 19
Example Output: a

Example Input: Problem: david works at a science lab that conducts experiments on bacteria . the population of the bacteria multiplies at a constant rate , and his job is to notate the population of a certain group of bacteria each hour . at 1 p . m . on a certain day , he noted that the population was 2,500 and then he left the lab . he returned in time to take a reading at 4 p . m . , by which point the population had grown to 160,000 . now he has to fill in the missing data for 2 p . m . and 3 p . m . what was the population at 3 p . m . ? 
Options: a ) 45,000 , b ) 40,000 , c ) 50,000 , d ) 55,000 , e ) 60,000
Example Output:
b