Task: Question Answering
Input (English): Problem: working simultaneously and independently at an identical constant rate , 6 machines of a certain type can produce a total of x units of product p in 5 days . how many of these machines , working simultaneously and independently at this constant rate , can produce a total of 4 x units of product p in 10 days ? 
Options: a ) 9 , b ) 10 , c ) 12 , d ) 14 , e ) 18
Output (English): c