主动脉瘤形成的主要原因是（）

A.感染

B.外伤

C.先天性畸形

D.中层囊性坏死

E.动脉粥样硬化

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

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

(2) 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.

(3) 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.

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

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

1（）

A．MD

B．RSA

C．DES

D．MIB