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If $C$ is a circle of radius $r$ with centre $z_{0},$ in the complex $z$-plane and if $n$ is a non-zero integer, then $\oint _{C}\frac{dz}{(z-z_{0})^{n+1}}$ equals

1. $2\pi n j$
2. $0$
3. $\frac{nj}{2\pi}$
4. $2\pi n$