In a central heating system, the seamless copper tubing used has an inside diameter of 20 mm and a wall thickness of 0.6 mm. For a maximum pressure of 1750 Kpa, when allowable stress is 225 Mpa, the safety factor will be

• A. 7.7
• B. 9.9
• C. 3.3
• D. 5.5
• E. Increasing the diameter of threaded portion