In this task, you need to provide the correct option for a given problem from the provided options.

Example Input: Problem: a certain hobby shop sold model cars for $ 5.00 each and model trains for $ 8.00 each . if a customer purchased both cars and trains from the store for a total of $ 31.00 , what total number of cars and trains did the customer purchase ? 
Options: a ) 3 , b ) 4 , c ) 5 , d ) 6 , e ) 7
Example Output: c

Example Input: Problem: there are 10 girls and 20 boys in a classroom . what is the ratio of girls to boys ? 
Options: a ) 1 / 2 , b ) 1 / 3 , c ) 1 / 5 , d ) 10 / 30 , e ) 2 / 5
Example Output: a

Example Input: Problem: the ratio between the present ages of a and b is 3.5 : 3 respectively . the ratio between a ' s age 4 years ago and b ' s age 4 years hence is 1 : 1 . what is the ratio between a ' s age 4 years hence and b ' s age 4 years ago ? 
Options: a ) 14 : 4 , b ) 13 : 0 , c ) 15 : 11 , d ) 13 : 12 , e ) 13 : 17
Example Output:
c