Task: Question Answering
Input (English): Problem: machines x and y produce bottles at their respective constant rates . machine x produces k bottles in 6 hours and machine y produces k bottles in 3 hours . how many hours does it take machines x and y , working simultaneously , to produce 12 k bottles ? 
Options: a ) 4 , b ) 8 , c ) 12 , d ) 18 , e ) 24
Output (English): e