With PGP， which of the following entity signs a users’s public key？（）

A. The sender of the message.
B. The receipient of the message.
C. The sender’s administrator who provides the sender with the PGP program.
D. A third party that belongs to what’s often known as "web of trust"， that can verify the relationship between the user and the key.
E. The vendor of the PGP program.