Describe the level curves f(x, y) = y - x.
◦ all straight lines in the xy-plane
◦ all straight lines in the xy-plane passing through the origin
◦ all straight lines in the xy-plane parallel to the line x + y = 2
◦ all straight lines in the xy-plane parallel to the line -x = y
◦ all straight lines in the xy-plane parallel to the line x = y