In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet. There are two types of key technology: private key and public key. Private - key encryption methods are called (66) ciphers, information is encrypted with (67) both the sender and receiver hold privately. Public - key encryption methods are called (68) ciphers, (69) created for each user, encryption key and decryption key ale (70) .

A．synchronous

B．Asynchronous

C．symmetric

D．asymmetric

有关下列各地美誉和美称的叙述，正确的是（  ）

A．塞外江南—西双版纳

B．春城—长春

C．日光城—拉萨

D．天府之国—江汉平原