Why is the address field in a PPP header always set to "11111111"?

Why is the address field in a PPP header always set to "11111111"?
A . PPP supports multipoint access networks with many stations. The all 1’s acts as a broadcast.
B . PPP supports multipoint access networks with many stations. The all 1’s acts as a multicast.
C . PPP supports point-to-point networks only. The address field is fixed.
D . PPP supports point-to-point networks only. The address field can change dynamically since every station is assigned a PPP address.

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Answer: C