Task: Question Answering
Input (English): Problem: working simultaneously and independently at an identical constant rate , 4 machines of a certain type can produce a total of x units of product p in 6 days . how many of these machines , working simultaneously and independently at this constant rate , can produce a total of 3 x units of product p in 6 days ? 
Options: a ) 24 , b ) 18 , c ) 16 , d ) 12 , e ) 8
Output (English): d