Which statement about Public Key Infrastructure (PKI) is true?

Which statement about Public Key Infrastructure (PKI) is true?

A.
PKI negotiates a secure connection between a client and a server.

B.
PKI specifies the use of a shared key.

C.
PKI uses digital certificates to bind public keys to an entity.

D.
PKI requires the use of a certificate authority and a registration authority.

Explanation:
http://www.comodo.com/resources/small-business/digital-certificates4.php