【单选题】
A Denial of Service (DoS) attack is:（）
A、an attempt to bring down a website or other online service.
B、a prank where someone sends an unauthorized e-mail attempting to close a user’s e-mail account.
C、stealing a user’s identity and making changes to their accounts.
D、an attempt to change a user’s password to prevent access to a service by repeatedly guessing the user’s information.
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