Task: Question Answering
Input (English): Problem: working alone , a can complete a certain kind of job in 6 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 ) 8 , b ) 10 , c ) 12 , d ) 14 , e ) 16
Output (English): c