Which of the following explains why this device is secure?

A zero-day vulnerability is discovered that affects a specific network device. This vulnerability only affects services on port 21. This network device is restricted to use only secure protocols and services .

Which of the following explains why this device is secure?
A . Because secure protocols would reject malicious packets
B . Because SCP is not a secure service
C . Because FTP is not a secure service
D . Because a secure firewall will protect the device

Answer: C