14 WiMAX（IEEE 802.16）MAC 子層協定提供了四種服務：(1)constant bit rate service、(2)real-time variable bit rate service、(3)non-real-time variable bit rate service、(4)best-efforts service，關於這四種服務有哪些是 connection-oriented，下列哪一項正確？ (A)只有 real-time variable bit rate service (B)只有 constant bit rate service 與 real-time bit rate service (C)這四種都不是 connection-oriented (D)這四種全都是 connection-oriented