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【单选题】
()是一个包含持有人、个人信息、公开密钥、证书序号、有效期、发证单位的电子签名等内容的数字文件。A.
（ A ）数字证书
B.
（ B ）数字签名
C.
（ C ）电子印章
D.
（ D ） CA
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举一反三
【单选题】The passage supplies information for answering which of the following questions?
A.
Where was the mechanism made?
B.
What was the primary purpose of the Antikythera mechanism?
C.
How did ancient scientists know how to use the Antikythera mechanism?
D.
Precisely how many gears did the mechanism have?
E.
What is the purpose of computer tomography?
【单选题】You’d better make a mark ______ you have any questions.
A.
at which
B.
at where
C.
the place
D.
where
【单选题】When you read books, you had better make marks ______ you have questions.
A.
where
B.
in where
C.
at which
D.
which
【简答题】What is the sky(天空)? Where is it? How high is it? What lies above it? These questions are hard to answer, aren’t they? Is the sky blue? The sky has no color. We know that there is air around the world...
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