How to put lines into same surface

Rhino Rhino for Mac
1

Screen Shot 2017-04-17 at 12.04.17 AM.png2440×996 43.6 KB

Hi guys. I m drawing line on the top view and try to extrend them. But I found out these lines do not on the same surface. Does anyone know Is there any way to put all the line on the same surface by a commend rather than by hand :frowning:

Thank you for the reply in advance !

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ProjectToCPlane (option Copy off) or SetPt (untick “Set X” and “Set Y”, then “Set Points” and select an object with the desired Z-coordinate.