MRI为了观察心房与心室的关系，主动脉根部及左室流出道应选哪种扫描位置（）

A.平行于室间隔的心脏长轴位

B.垂直于室间隔的心脏长轴位

C.横轴位

D.矢状位

E.垂直于室间隔的心脏短轴位

Frame Relay is simplified form of (25) , similar in principle to (26) , in which synchronous, frames of data are routed to different destinations depending on header information . Packets are routed throught one or more Virtual Circuits known as (27) . Most Virtual Circuits are (28) ,which means that the network provider sets up all DLCI connections at subscription time. (29) are also part of the Frame Relay specification . They privide a link that only lasts only as long as the session.

26（）

A．X.21

B．X.25

C．X.28

D．X.29