患者男性，40岁，多次发作心悸，心电图见图3-16-8，显示为预激图形。

食管电生理检查测得旁路前传有效不应期为240ms，提示（）。

A.低危旁路

B.高危旁路

C.非危险旁路

D.可能危险

E.以上都不是

Even though current technologies make the ultimate segmented network impractical, new technologies and products hold the promise of making the concept a reality. Most important of all these new technologies are (66) . The ultimate switched LAN, which is technically possible, now, exhibits the following characteristics:
1. High-speed yet cost-effective switching devices can be implemented on a wide scale, providing dedicated connections for individual users and supporting the aggregate (67) of all networked users.
2. The ability to deploy switching capabilities incrementally into the (68) base. Switching implementations support switching of individual users or groups so that existing users of shared media modules can continue to use the existing equipment until they can justify moving to dedicate switched LAN connections.
3. Support and enhancement of existing routed environments. Switching (69) routint, but rather can be introduced into the network without making any changes to the existing routing equipment or configuration. (70) play a critical role in the integration of switching into existing networks.

A．switch-based LANs

B．WANs

C．user services

D．LAN-BY-PHONE