What Is the Difference Between Unavailable and High Availability

What is the difference between unavailable and high availability?

A) High availability is when the system is not operating and can't be used, whereas unavailable is when a system is continuously operating at all times.
B) Unavailable only refers to clients, whereas high availability refers to servers.
C) Unavailable is when the system is not operating and can't be used, whereas high availability is when a system is continuously operating at all times.
D) High availability only refers to clients, whereas unavailable refers to servers.