Macintosh Key Commands https://support.apple.com/en-ca/HT201255
Shift ⇧ Start up in Safe Mode. Option ⌥ Start up to Startup Manager. C Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media). D Start up to either Apple Hardware Test or Apple Diagnostics, depending on which Mac you're using. Option-D Start up to either Apple Hardware Test or Apple Diagnostics over the Internet. N Start up from a compatible NetBoot server. Option-N Start up from a NetBoot server using the default boot image. Command (⌘)-R Start up from OS X Recovery. Command-Option-R Start up from OS X Recovery over the Internet. Command-Option-P-R Reset NVRAM. Release the keys after you hear the startup sound again. Command-S Start up in single-user mode. T Start up in target disk mode. X Start up from an OS X startup volume when the Mac would otherwise start up from a non-OS X startup volume. Command-V Start up in verbose mode. Eject (⏏), F12, mouse button, or trackpad button Eject removable media, such as an optical disc.