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.

A．binarytext

B．hypertext

C．plaintext

D．supertext

控制景物曝光范围在胶片（）以内，是曝光控制的基本要点之一。

A、感光度

B、宽容度

C、感色性

D、反差性