Calculate circumferential prestress of a cylindrical prestressed concrete water tank given that the thickness is 12mm, loss ratio is 0.75, the maximum stress under working pressure is 1n/mm2(Nd value is 720)?

• A. 9.4n/mm2
• B. 5.6n/mm2 c) 11.2n/mm2 d) 15.2n/mm2