【简答题】

试题12
    从供选择的答案中，选出应填入下面叙述中_?_内的最确切的解答，把相应编号写在答卷的对应栏内。
    In a public-key cryptosystem, _A_ encrypted with the public key can only be deciphered with the private key from the same _B_. Conversely, _A_ encrypted with the private key can be decrypted only with the public key. Users can make their public keys _C_ available or place them at a key distribution center or others to access. However, the private key must be _D_ safe. In public-key systems there’s no _E_ to find a safe channel for communicating a shared secret key.
A：①binarytext        ②hypertext        ③plaintext        ④supertext
B：①length            ②pair            ③size            ④space
C：①free            ②freely        ③locally        ④partly
D：①keep            ②keeped        ③keeping        ④kept
E：①more            ②necessary        ③need            ④space