Yes, I had the same experience.  There is a one-sentence paragraph late in the lesson that says that *rarely* a period is used rather than a space.  There is not a single example of this in the entire lesson.  I think this question needs to be reconsidered.  It has the effect of emphasizing the exception, teaching the student that one can equally well write either a space or a period (full stop for Brits), when in fact one hardly ever sees anything but a space and you’ll get funny looks if you use the period.  The student will only realize that the period isn’t just as good as the space if they read every word of the lesson.  I think this is a bad result, and the question should be reworked.