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I made horizontal-supposed lines using line tool and holding Shift key. But it seems like my lines are not completely horizontal. I am attaching the capture of what I see.
This becomes more obvious when I zoom in.
Check the lines in Outline Mode by choosing View > Outline. If they are straight there, then the issue is with the pixel display of the monitor you are using and its inability to accurately place pixels for the preview.
View > Outline
Required, but never shown