Which statement is true about a certificate authority (CA)?

Which statement is true about a certificate authority (CA)?

A.
A trusted third party responsible for signing the private keys of entities in a PKIbased system

B.
A trusted third party responsible for signing the public keys of entities in a PKIbased system

C.
An entity responsible for registering the private key encryption used in a PKI

D.
An agency responsible for granting and revoking public-private key pairs