A.3 B.4 C.5 D.6
A.數(shù)字?jǐn)?shù)據(jù)網(wǎng) B.綜合數(shù)字網(wǎng) C.數(shù)據(jù)通信分組網(wǎng) D.公用數(shù)據(jù)網(wǎng)
A.三小時(shí) B.兩小時(shí) C.一小時(shí) D.半小時(shí)