A, the step of cracking the root password system centos 7
(a) loading the mirroring system
(b) alter the code
(C) exit to exit and restart
1. After restarting the system, deftly clicking in the loading process "esc" key to enter the boot menu 
2. Next choose to solve difficult problems, diagnostic system options.
3. load the image, change the password
chroot / mnt / sysimage / 
4.eixt exit bash
5.reboot restart