Which of the following is false about PKI?

a$.$ It is the underlying infrastructure that serves as a key management system for controlling public keys, private keys, and digital certificates.

b$.$ It is the set of software, hardware, processes, procedures, and policies that are needed to create, manage, distribute, use, store, and revoke digital certificates across large user populations.

c$.$ It is digital certificate management at scale.

d$.$ It must be used by all enterprises with over $1,000$ employees