In this task, you need to answer the given multiple-choice question on the general math. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Example Input: Problem: how many positive integers less than 100 have a remainder of 6 when divided by 13 ? 
Options: a ) 2 , b ) 7 , c ) 6 , d ) 9 , e ) 10
Example Output: c

Example Input: Problem: a certain number of horses and an equal number of men are going somewhere . half of the owners are on their horses ' back while the remaining ones are walking along leading their horses . if the number of legs walking on the ground is 40 , how many horses are there ? 
Options: a ) 8 , b ) 12 , c ) 14 , d ) 16 , e ) 18
Example Output: a

Example Input: Problem: if a 2 - b 2 = 10 and a * b = 5 , find a 4 + b 4 . 
Options: a ) 50 , b ) 100 , c ) 150 , d ) 92 , e ) 25
Example Output:
c