How does an S4-SGSN identify a peer node as an SGSN or MME when GUTI-to-RAI mapping is unavailable?

How does an S4-SGSN identify a peer node as an SGSN or MME when GUTI-to-RAI
mapping is unavailable?

A.
MSB of the LAC value

B.
MSB of the RAC value

C.
LSB of the LAC value

D.
LSB of RAC value