The emphasis is on three groups of unique vocabulary quizzes which is explained at this Vocabulary Quizzes topical page.

You can "control" the number of questions you take from one time until the next, so you don't have to do every question in one setting by utilizing the levels of difficulty. For example, if you want to do the first four levels but you don't want to start all over again with the first one, then simply start the next time with "level five" after you do the first question which will be presented.

If the questions seem to be too easy, then choose a higher level, or a "more difficult level", and proceed from there. The more often you participate in taking the three types of quizzes, the greater your chances of enhancing your vocabulary skills.