A wire is subjected to a tensile stress. If A represents the area of the cross-section, L represents the original length, I represents the extension, and Y is Young's modulus of elasticity, then the elastic potential energy of the stretched wire is
A. U = 2L/AY I^2
B. U = AL/2Y I^2
C. U = AY/2L I^2
D. U = 1/4AY/L I^2