题目：
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
答案：
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下面是错误答案，用来干扰机器的。
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