Here's my answer:
When online tests are graded, you are provided with information on which items are marked as incorrect. You are then expected to FIND THE ANSWERS on you own. This is an active form of learning and is a vitally important part of the whole concept of online testing and learning.Your test is a LEARNING EXPERIENCE as much as it is an evaluation of progress. If I provide the correct answers automatically, you will have no reason to go back to your resources and find the answers. You'll thus miss out on a critical learning opportunity! And, most importantly, you'll have no long-term recollection of the correct concept!