Task: Question Answering
Input (English): Problem: working alone , pump a can empty a pool in 4 hours . working alone , pump b can empty the same pool in 6 hours . working together , how many minutes will it take pump a and pump b to empty the pool ? 
Options: a ) 132 , b ) 138 , c ) 144 , d ) 150 , e ) 156
Output (English): c