A)     Natural disaster
B)     Human error
C)     Hardware failure
D)     Software failure 

Although natural disasters like hurricanes, tornadoes, and earthquakes get all the press, they actually cause a mere 5% of all downtime. 

Hardware failure is by far the number one cause of network downtime and lost productivity at small to mid-sized organizations. Even for organizations that have multiple levels of redundancy in their storage networks, redundant power supplies, and virtual host failover, it only takes one ill-timed cable cut, broken switch port, or simultaneous drive failures to take down a network.