In this key establishment protocol, 

• interacts with a trusted server and a party .  
• and share a symmetric key , and and share a symmetric key .  
• will choose a key to be shared by and .
• and are random numbers chosen by and respectively.
• We will write to mean encrypting a message using a symmetric key encryption algorithm with key .

The protocol runs as follows:

1. :  , , .
2. : .
3. : .
4. : .
5. : .

Which message, if any, provides confirmation to that the key is possessed by  ?  Pick only one answer.