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.