Isolate a video problem as hardware or software on a Mac

To isolate a video problem as hardware or software on a Mac, which of the following steps should you try FIRST?
A. Replace the built-in display.
B. Reinstall the appropriate Mac OS.
C. Verify the hard disk with Disk First Aid.
D. Start up from a known-good startup volume.
[toggle title_open=”Close Me” title_closed=”Answer” hide=”yes” border=”yes” style=”default” excerpt_length=”0″ read_more_text=”Read More” read_less_text=”Read Less” include_excerpt_html=”no”]D[/toggle]