For a six–sigma spread in the distribution of the product quality characteristic, the upper natural control limit will fall at _____________$

• A. ‚âà√≠¬¨‚à´ + 2‚âà√¨‚àö√¢
• B. ‚âà√≠¬¨‚à´ + 3‚âà√¨‚àö√¢
• C. ‚âà√≠¬¨‚à´ + 4‚âà√¨‚àö√¢
• D. ‚âà√≠¬¨‚à´ + 6‚âà√¨‚àö√¢