Melissa and LoveLetter made use of the trust that exists between friends or colleagues.Imagine receiving an （71） from a friend who asks you to open it. This is what happens with Melissa and several other similar email （72） . Upon running, such worms usually proceed to send themselves out to email addresses from the victim’s address book, previous emails, web pages （73） .As administrators seek to block dangerous email attachments through the recognition of well-known （74） , virus writers use other extensions to circumvent such protection.Executable (.exe) files are renamed to .bat and .cmd plus a whole list of other extensions and will still run and successfully infect target users.Frequently, hackers try to penetrate networks by sending an attachment that looks like a flash movie, which, while displaying some cute animation, simultaneously runs commands in the background to steal your passwords and give the （75） access to your network.

A．cracker

B．user

C．customer

D．client

反向组合中，检波器成对反接形式多样，可以是（）。

A.对称于中心点

B.不对称

C.正负相间