Which of the following is NOT correct about high availability across zones?

a$.$ In a cloud computing environment, reliability and resiliency are achieved through duplicating processes across one or more geographical areas.

b$.$ An Availability Zone $($AZ$)$ is one or more data centers within a Region, each with redundant power, networking, and connectivity.

c$.$ They are more highly available, fault tolerant, and scalable than would be possible with a single data center.

d$.$ They require that specific security appliances be located on$-$prem so that the local data center can be considered as a qualified Zone.