Which of the following equations has NO positive integer solutions?
x4+y4=z2x^4 + y^4 = z^2x4+y4=z2
x2+y2=5x^2 + y^2 = 5x2+y2=5
x2+2y2=11x^2 + 2y^2 = 11x2+2y2=11
x3+y3=2z3x^3 + y^3 = 2z^3x3+y3=2z3