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 swimmer can swim in still water at 4 km / h . if the speed of the water current is 2 km / h , how many hours will the swimmer take to swim against the current for 5 km ?
Options: a ) 2.0 , b ) 2.5 , c ) 3.0 , d ) 3.5 , e ) 4.0
Example Output: b

Example Input: Problem: what is the length of a bridge ( in meters ) , which a train 156 meters long and travelling at 45 km / h can cross in 30 seconds ?
Options: a ) 203 , b ) 211 , c ) 219 , d ) 227 , e ) 235
Example Output: c

Example Input: Problem: one copy machine can make 15 copies a minute , and a second copy machine makes 10 copies a minute . if the two copiers work together , how long would it take them to make 1,000 copies ?
Options: a ) 20 minutes , b ) 30 minutes , c ) 40 minutes , d ) 50 minutes , e ) 60 minutes
Example Output:
c