In an open die forging, a circular disc is gradually compressed between two flat platens. The exponential decay of normal stress on the flat face of the disc, from the center of the disc towards its periphery, indicates that

• A. there is no sticking friction anywhere on the flat face of the disc
• B. sticking friction and sliding friction co-exist on the flat face of the disc
• C. the flat face of the disc is frictionless
• D. there is only sticking friction on the flat face of the disc