Two routers, connected together through serial ports, However, the interconnection cannot be pinged. Now check the port status as follows

Can you identify the cause from the above information? (Single Choice)
- Insufficient link bandwidth
- IP addresses at both ends are not on the same network segment
- Inconsistent link layer protocols at both ends
- Subnet mask mismatch
Answer(s): C
Explanation:
Different link layer protocols, R1 is HDLC, R2 is PPP
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