A harmonic function is analytic if it satisfies the Laplace equation. If $u(x,y)=2x^2-2y^2+4xy$ is a harmonic function, then it is conjugate harmonic function $v(x,y)$ is
1. $4xy-2x^2+2y^2+ \text{constant}$
2. $4y^2-4xy + \text{constant}$
3. $2x^2-2y^2+ xy + \text{constant}$
4. $-4xy+2x^2-2y^2+ \text{constant}$