下列有关血红蛋白结构的概念哪项是错误的（）

A.血红蛋白是珠蛋白和亚铁血红蛋白组合的蛋白质

B.每个血红蛋白分子含4个亚铁血红蛋白

C.亚铁血红蛋白由原卟啉和一个Fe组成

D.每一个亚铁血红蛋白包裹一个珠蛋白

E.珠蛋白的肽链分为α、β两类

The network layer provides services to the transport layer. It can be based on either（71）. In both cases，its main job is （72） packets from the source to the destination. In network layer，subnets can easily become congested，increasing the delay and（73）for packets. Network designers attempt to avoid congestion by proper desing,Techniques include （74） policy，caching，flow control，and more. The next step beyond just dealing with congestion is to actually try to achieve a promised quality of service. The methods that can be used for this include buffering at the client，traffic shaping，resource（75），and admission control. Approaches that have been designed for good quality of service include integrated services (including RSVP), differentiatd services, and MPLS.

空白处(72)应选择（）

A．dealing with

B．routing

C．sending

D．receiving