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Bring out the review panel. Review the comments. Add your own comments. Reply to a comment.
While viewing the file you opened, locate the chat icon at the upper-right menu of the window. Hovering on it will pop up the texts "Show or hide comments"; click on it. The review panel will flow from the right. On the review panel, you can see all the comments left by the collaborators of the file. The names of the people who left reviews, together with the date and time stamps and their actual comments, can be seen. If you have something to add, click on the "Enter a Comment" button. A message box will appear where you can type in your comments. When you're done, click on the "Add Comment" button right below the message box. Your comment will appear right away. If you want to reply to an existing comment, just hover over the comment you want to reply to and a Reply text will appear. Click on this and a message box will appear. Type in your response and click the "Add Reply" button right below the message box. Your response will appear right away.