If x longitudinal strain is produced in a wire of Young's modulus y, then energy stored in the material of the wire per unit volume is [MP PMT 1987, 89, 92; CPMT 1997; Pb. PMT 1999; KCET 2000; AIIMS 2001]

• A. \[y{{x}^{2}}\]
• B. \[2\,y{{x}^{2}}\]
• C. \[\frac{1}{2}{{y}^{2}}x\]
• D. \[\frac{1}{2}y{{x}^{2}}\]