Why is the need for safety stock reduced?

• A. Because when demand at one site goes up, demand at the other sites tends to go down.
• B. Because the chance than demand will be high (or low) at all sites at the same time is small.
• C. Because when demand at one site goes up, demand at the other sites tends to go up as well.
• D. Because the standard deviation of the sum is less than the sum of the standard deviations.