what is the Next Hop address of MP-Ibgp Update for CE4 network received at PE-R2 ?

In the Inter-AS VPN deployment shown in the diagram, what is the Next Hop address of MP-Ibgp Update for CE4 network received at PE-R2 ?

A.
PE-R4 50.0.0.4

B.
ASBR-1 50.0.0.1

C.
ASBR-2 50.0.0.3

D.
PE-R2 50.0.0.2