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'.

Ex Input:
Problem: the population of a bacteria colony doubles every day . if it was started 8 days ago with 3 bacteria and each bacteria lives for 12 days , how large is the colony today ?
Options: a ) 512 , b ) 768 , c ) 13122 , d ) 2048 , e ) 4096

Ex Output:
c

Ex Input:
Problem: a and b together can do a work in 4 days . if a alone can do it in 20 days . in how many days can b alone do it ?
Options: a ) 11 , b ) 5 , c ) 6 , d ) 8 , e ) 25

Ex Output:
b

Ex Input:
Problem: a man started driving at a constant speed , from the site of a blast , the moment he heard the blast . he heard a second blast after a time of 30 mins and 20 seconds . if the second blast occurred exactly 30 mins after the first , how many meters was he from the site when he heard the second blast ? ( speed of sound = 330 m / s )
Options: a ) 4200 , b ) 5400 , c ) 6600 , d ) 7800 , e ) 8900

Ex Output:
c