In the protocol of Fig. 8-39, why is A sent in plaintext along with the encrypted session

Question:

In the protocol of Fig. 8-39, why is A sent in plaintext along with the encrypted session key?
In the protocol of Fig. 8-39, we pointed out that starting each plaintext message with 32 zero bits is a security risk. Suppose that each message begins with a per-user random number, effectively a second secret key known only to its user and the KDC. Does this eliminate the known plaintext attack? Why?