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

Example Input: Problem: one lady has 2 children , one of her child is boy , what is the probability of having both are boys 
Options: a ) 1 / 2 , b ) 1 / 3 , c ) 1 / 4 , d ) 1 / 5 , e ) 3 / 4
Example Output: a

Example Input: Problem: a company sells pens and pencils . the revenue from pens in 2007 was up 5 % from 2006 . the revenue from pencils declined 13 % over the same period . overall revenue was down 1 % from 06 to 07 . what was the ratio of pencil revenue to pen revenue in 2006 
Options: a ) 1 / 2 , b ) 1 / 3 , c ) 1 / 4 , d ) 1 / 5 , e ) 1 / 6
Example Output: a

Example Input: Problem: a can contains a mixture of two liquids a and b in the ratio 7 : 5 . when 9 litres of mixture are drawn off and the can is filled with b , the ratio of a and b becomes 7 : 9 . how many litres of liquid a was contained by the can initially ? 
Options: a ) 25 , b ) 21 , c ) 20 , d ) 10 , e ) 15
Example Output:
b