If d is the diameter of the pipe, p is the total internal pressure, f is the permissible tensile stress and n is the effective of the joint, the thickness t of metal pipe, is

• A. <img src="/_files/images/civil-engineering/water-supply-engineering/221-10.253-1.png">
• B. <img src="/_files/images/civil-engineering/water-supply-engineering/221-10.253-2.png">
• C. <img src="/_files/images/civil-engineering/water-supply-engineering/221-10.253-3.png">
• D. <img src="/_files/images/civil-engineering/water-supply-engineering/221-10.253-4.png">
• E. none of these.