Or even selecting all and open Properties Palette. Then change lines start and
end point Z values from *varies* to 0.0. If they're polylines, change their elevation to 0.0. Flatten could cause some problems with text and blocks, so be careful. If you don't mind having lines in 3d, but you want your own drawing to be on a 2d plane. Open the Options Dialog (Tools --> Options), then on the ...