在采用送票制的预订客票情况下，()为要约。

A.旅客取票并支付票款的行为

B.承运人送票的行为

C.旅客买票的行为

D.旅客招拦出租车的意思表示

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 (71) ciphers, information is encrypted with (72) both the sender and receiver hold privately. Public-key encryption methods are called (73) ciphers, (74) created for each user, encryption key and decryption key are (75)

A．synchronous

B．Asynchronous

C．symmetric

D．asymmetric