Answer:
If someone sends an email to more than one person, you have two option when replying. Clicking "Reply" sends your message to the email's sender, while clicking "Reply All," sends your message to everyone who received the original.
Hitting Reply All ensures the Cc'd person receives future emails that are part of this thread. If your response will influence other people's decision making or current thread recipients should know others are looped in, go with Cc or Reply All.
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