Which box did she open and how did she know?

Kelly opened the mixed fruit box first. She knows because the fruit she pulled had a label on the box that matched the fruit that she pulled. If She knows the labels are incorrect then she would know that it was not the apple box if she pulled an apple. She would then know that the box that says mixed would be the orange box because the apple label was not available as it was on the box that she pulled the apple from. That would leave the orange label to remain on the apple box.