In a public-key cryptosystem, (71) encrypted with the public key can only be deciphered with the private key from the same (72) . Conversely, (71) encrypted with the private key can be decrypted only with the public key. Users can make their public keys (73) available or place them at a key distribution center or others to access. However, the private key must be (74) safe. In public-key systems there’s no (75) to find a safe channel for communicating a shared secret key.

（72）是（）

A.length

B.pair

C.size

D.space

“过邯郸闻贵妇人即为带下医过雒阳闻周人爱老人即为耳目痹医来入咸阳闻秦人爱小耳即为小儿医随俗为变”在“小儿医后”的标点是（）

A.逗号

B.句号

C.冒号

D.分号