Task: Question Answering
Input (English): Problem: 20 one identical machines, running continuously at the same constant rate, take 9 days to complete a shipment. how many additional machines, each running at the same constant rate, would be needed to reduce the time required to complete a shipment by two days ?
Options: a. 2, b. 3, c. 4, d. 6, e. 9
Output (English): d