Task: Question Answering
Input (English): Problem: working alone , a can complete a certain kind of job in 12 hours . a and d , working together at their respective rates , can complete one of these jobs in 4 hours . in how many hours can d , working alone , complete one of these jobs ? 
Options: a ) 1 , b ) 3 , c ) 4 , d ) 6 , e ) 9
Output (English): d