Which of the following equations is a 'linear' Diophantine equation?
15x+10y=515x + 10y = 515x+10y=5
x2+y2=z2x^2 + y^2 = z^2x2+y2=z2
xy=12xy = 12xy=12
y2=x3+1y^2 = x^3 + 1y2=x3+1