Yeah, the K&B book warned me regarding this problem earlier, so I took a safety precaution of not answering the "drag and drop" until I finished with the multiple-choices questions. =)
Or if you really tempt to answer the drag and drop questions first - then either you answer them correctly (and confidently) OR choose not to answer it at all just yet (meaning no partial solution and revise later technique), so that you won't get your previous answers deleted while trying to view it again later.
And yeah, the pop-up warning message should be helpful aid for you of not mistakenly revise the drag-and-drop question which was already answered earlier.
