Which of the items stated is typically the most expensive method of improving QoS across WAN links？（）

A. platinum， gold， silver， and bronze
B. voice， gold， silver， and bronze
C. low delay， network critical， business， best effort
D. voice， video， background gold， background silver， and best effort
E. voice， video， best effort， and background
F. voice， video， best effort， background， and scavenger

A. Lower-priority traffic is assigned short interframe wait-timers to allow higher-priority traffic access to the wireless network first.
B. Packets that are not assigned to a specific access category are categorized by default as the background access category.
C. The WMM standard was implemented after the 802.11e standard was approved to provide support for non 802.11e devices.
D. WMM is a Wi-Fi Alliance standard that was implemented before the 802.11e standard was approved.
E. WMM uses Enhanced DCF （EDCF） to stipulate fixed and random wait times for the different access classes.

A. gold， silver， bronze， and platinum
B. platinum， gold， silver， and bronze
C. titanium， gold， silver， and bronze
D. voice and video， best effort， background， and scavenger
E. voice， video， best effort， and background
F. voice， video， best effort， and scavenger