第20题: [单项选择]Which of the following statement about the depression of our ancestors is NOT true A. make us feel miserable B. last longer than two weeks C. make it difficult for us to do things D. end until we go to see a therapist 参考答案:B 答案解析:原文 common depression... may last from several hours to several days. 因此[B]为正确答案。
第21题: [单项选择]数字签名是使用() A. 自己的私钥签名 B. 自己的公钥签名 C. 对方的私钥签名 D. 对方的公钥签名 参考答案:A