题目：
A glue that holds the whole Internet together is the network layer protocol, (71)．Unlike most older network layer protocols, it was designed from the beginning with internetworking in mind．Its job is to provide a (72) way to transport datagrams from source to destination，without regard to whether these machines are on the same network or whether there are other networks in between them． Communication in the Internet works as follows．The (73) layer takes data streams and breaks them up into datagrams．Each datagram is transmitted through the Internet, possibly being fragmented into smaller units as it goes．When all the pieces finally get to the destination machine, they are reassembled by the (74) layer into the original datagram．This datagram is then handed to the transport layer, which inserts it into the receiving process’ input stream． An IP datagram consists of a header part and a text part．The header has a (75) part and a variable length optional part．
空白处（75）应选择（）
A.40-byte fixed
B.64-byte fixed
C.20~64 bytes variable
D.20-byte fixed
答案：
参考答案：D
解析：
参考译文将整个互联网连成一体的是网络层协议______(71)。与大多数更早的网络层协议不同，它在设计之初就充分考虑了网络互连。它负责提供一种 _______(72)的方式，从信源到信宿传递数据报，不管信源和信宿机器是否在同一个网络中，或者它们之间是否有其他网络。互联网中的通信遵照以下方 式进行。_______(73)层将数据流分割成数据报，每个数据报通过互联网传输的过程中，有可能被分割成更小的单元。当所有的数据单元最终到达目的地 主机时，它们被_________(74)层重新组合成原始数据报。这个数据报随后被提交给传输层，由传输层将其插入接收进程的输入流中。一个IP数据报 包含报头和报文两部分。报头包括一个_________(75)部分和一个可变长度的可选部分。
