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: village x has a population of 72000 , which is decreasing at the rate of 1200 per year . village y has a population of 42000 , which is increasing at the rate of 800 per year . in how many years will the population of the two villages be equal ? 
Options: a ) 15 , b ) 19 , c ) 11 , d ) 18 , e ) 13
Example Output: a

Example Input: Problem: ? x 24 = 173 x 240 
Options: a ) 545 , b ) 685 , c ) 865 , d ) 1730 , e ) 534
Example Output: d

Example Input: Problem: elena purchased brand x pens for $ 4.00 apiece and brand y for $ 2.20 apiece . if elena purchased a total of 12 of these pens for $ 42.00 , how many brand x pens did she purchase ? 
Options: a ) 4 , b ) 5 , c ) 6 , d ) 7 , e ) 8
Example Output:
c