As you should have read on the page that appears before starting the test, you will have to copy the question here if you want me to show the workings.
The reason is (as is explained on that page) that in the section A questions are selected at random from a large bank of questions. So what came up for you as question 21, will not come as question 21 if you attempt the test again. So question 21 does not mean anything to me – in the bank of questions it could be anywhere 🙂