能阻碍血红蛋白携O₂，但不阻碍氧合血红蛋白释O₂的是

A．酸度增加
B．酸度降低
C．Hb的Fe²⁺氧化成Fe³⁺
D．低氧
E．CO中毒

Networks can be interconnected by different devices. In the physical layer, networks can be connected by （66） or hubs, which just move the bits from one network to an identical network. One layer up we find bridges and switches, which operate at data link layer. They can accept （67） , examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two networks. If two networks have （68） network layer, the router may be able to translate between the packet formats. In the transport layer we find transport gateway, which can interface between two transport connections. Finally, in the application layer, application gateways translate message （69） . As an example, gateways between Internet e-mail and X.400 e-mail must （70） the e-mail message and change various header fields.

（67）处填（）。

A．frames

B．packets

C．packages

D．cells