It's not very clear, but the bit after the ":" is compulsory because this holds the explanation for the correct answer to be shown. The ":" is not a flag to show the correct answer to the user, it is a way to mark-up an explanation that will be shown with the correct answer. Slightly different: it's either the correct answers are not shown, or they are shown along with the given explanations. I have clarified the language for the next patch.