What should be the minimum thickness of pipe if the maximum stress is not to exceed 80 N/mm2?

Question

A seamless pipe of diameter 1m is to carry fluid under a pressure of 2 N/mm². What should be the minimum thickness of pipe if the maximum stress is not to exceed 80 N/mm²?

         (a) 12.5 mm        (b) 10 mm        (c) 15 mm        (d) 7.5 mm