Which statement is true about IP telephony calls？（）

• A、 A Voice over IP （VoIP） packet consists of the voice payload， IP header， TCP header， RTP header， and  Layer 2 link header.
• B、 The voice carrier stream uses H323 to set up， maintain， and tear down call endpoints.
• C、 Call control signaling uses Real-Time Transport Protocol （RTP） packets that contain actual voice  samples.
• D、 The sum of bandwidth necessary for each major application， including voice， video， and data， should not exceed 75 percent of the total available bandwidth for each link.