If a CentOS 7 administrator needs to review the amount of time since the last system reboot and how many users are logged on, what command can they use?

uptime

Explanation:

uptime command gives information а bout:

current time the amount of time since the last system reboot How many users are currently logged on The system load averages for the past 1, 5, and 15 minute periods

A sample output is like this:

19:03:17 up 9:40, 1 user, load average: 0,20, 0,28, 0,26

which means:

19:03:17 is the current time the system is up since 9 hours 40 min. 1 user is logged on The system load averages for the past 1, 5, and 15 minute are 0,20, 0,28, 0,26