How to delete one kind of music file and keep another.

Hello everyone, I'm new here and I hope you can help me. I have an iPod touch, and I am trying to add my music to it, but I only want the AAC versions on my iPod. I've got some mp3 tracks in iTunes, and some AAC versions as well. I have added the "kind" column, to order the files by their mp3 and AAC versions. Now I want to delete the mp3 versions, and keep the AAC versions to add to my iPod. Is there a way of highlighting the mp3 versions en masse, so I can select all the mp3 tracks in one go, ready to delete them? I don't want to remove them from the computer itself; just from iTunes. I could select each mp3 track in turn, and delete them individually, but that is a laborious way of doing the job. I'm looking for an easier method. Can anyone offer me a suggestion as to how to do this please, either via iTunes, or via a file management method in my Windows PC? Thank you for your time in reading this question.