Circles and Arcs in SketchUp

Just getting started on the free version of sketchup.   I'm following your tutorial - day 2, and when I erase the lines below the arc window, it erases the entire wall, versus just the line.  Not sure you could assist?  This must be a pretty simple thing i'm overlooking, because I tried looking for the error, and couldn't find anything.. any help is appreciated.  Thanks!

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