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: two trains t 1 and t 2 start simultaneously from two stations x and y respectively towards each other . if they are 70 km apart both 3 and 6 hours after start , then find the distance between the two stations . 
Options: a ) 210 km , b ) 240 km , c ) 220 km , d ) 180 km , e ) 190 km
Example Output: a

Example Input: Problem: a can do a piece of work in 4 hours ; b and c together can do it in 2 hours , while a and c together can do it in 2 hours . how long will b alone take to do it ? 
Options: a ) 12 , b ) 9 , c ) 8 , d ) 6 , e ) 4
Example Output: e

Example Input: Problem: the population of a bacteria colony doubles every day . if it was started 9 days ago with 2 bacteria and each bacteria lives for 11 days , how large is the colony today ? 
Options: a ) 512 , b ) 768 , c ) 4096 , d ) 2048 , e ) 1024
Example Output:
e