Within the WMM policy， how many default QoS radio access categories are there？（）

A. Resource Reservation Protocol （RSVP）
B. first-in， first-out （FIFO）
C. best-effort
D. CBWFQ
E. IntServ
F. DiffServ

A. Packets can arrive out of order.
B. Packets must take the same path to the destination.
C. Once packets are received at the destination router， the router performs sampling to ensure the quality of the voice traffic.
D. Packets can experience varying delays across the network.
E. Packets containing voice samples are sequentially numbered by default， allowing for reordering upon arrival at the terminal router along the path.

A. Compress payload.
B. Use header compression.
C. Increase link capacity.
D. Use random early detection.
E. Guarantee bandwidth to fragile flows.
F. Implement network-based application recognition.