Which possible cause is true?

7.21 configured ipv4, authenticated instance invalid, unsynced, stratum 16

ref ID INIT, time 00000000 0000000 (17:00:00.000 ccie Wed Dec 31, 1899)

R2 is getting time synchronized from NTP server R1. It has been reported that clock on R2 cannot associate with the NTP server R1.

Which possible cause is true?
A . R2 has connectivity issue with the NTP server
B . R1 has an incorrect NTP source interface defined
C . R2 should not have two trusted keys for the NTP authentication
D . R2 does not support NTP authentication
E . R2 has an incorrect trusted key binded with the NTP server
F . R2 has incorrect NTP server address

Answer: B