A thin cylindrical shell 500 mm in diameter and 25 mm thick having $E=2.0\times 10^{5}N/mm^{2}$ and $\mu =0.25$,
is subjected to internal fluid pressure of 20 N/mm2 . What is unit change in volume of the shell?
(a) $4\times 10^{-3}$
(b) $2.5\times 10^{-3}$
(c) $2\times 10^{-3}$
(d) $1.5\times 10^{-3}$