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For the function of a complex variable $W=\ln Z$ (where, $\mathrm{W}=u+j \mathrm{v}$ and $\mathrm{Z}=x+j y),$ the $u=$ constant lines get mapped in $Z$-plane as

1. set of radial straight lines
2. set of concentric circles
3. set of confocal hyperbolas
4. set of confocal ellipses