[单项选择]A branch office of Acme Insurance has 15 IP phones connected to the main office using the distributed call processing model. Normally， the phones work with great quality. However， during very busy times when all of the agents are on the phone at the same time， the voice quality drops precipitously. Words or phrases or both are dropped from conversations. What is the most likely cause of the problem？（）
A. large files are being downloaded over the WAN network
B. IP phone traffic is not being classified correctly
C. more bandwidth is required for the office LAN
D. Call Admission Control has not been implemented 
E. employees are watching videos over the internet
F. header compression is not being used