下列有关生物群落的叙述，正确的是[ ]

A．群落演替过程中各种群呈“S”型增长

B．群落的稳定性与群落中物种丰富度呈正相关

C．群落内的种间竞争比种内竞争剧烈

D．群落中垂直结构的形成有利于自然资源的充分利用

For each blank, choose the best answer from the four choices and write down on the answer sheet.

(66) is a one-way function that takes an arbitrarily long piece of plaintext and from it computes a fixed-length bit string.

(67) is a protocol that most USENET machines now use it to allow news articles to propagate from one machine to another over a reliable connection.

(68) is an approach preferred by cable TV providers. In this solution the fiber terminates at an optoelectrical converter and the final segment is a shared coaxial cable.

(69) is a device used to connect two separate network that use different communication protocol.

(70) is a digital-to-digital polar encoding method in which the signal level is always either positive or negative.

70（）

A．NNI

B．NRZ

C．NAK

D．NVT