If is the deviation of the individual test strength from the average of n samples, the standard deviation of concrete of the given grade is

• A. <img src="/_files/images/civil-engineering/structural-design-specifications/01-22-34-1.png">
• B. <img src="/_files/images/civil-engineering/structural-design-specifications/01-22-34-2.png">
• C. <img src="/_files/images/civil-engineering/structural-design-specifications/01-22-34-3.png">
• D. <img src="/_files/images/civil-engineering/structural-design-specifications/01-22-34-4.png">