Subject: mathqa
Question: working simultaneously and independently at an identical constant rate , 20 machines of a certain type can produce a total of x units of product p in 4 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 8 days ?
Option: 
A: 40
B: 35
C: 32
D: 30
Answer: D