Mathematics teacher: Teaching students calculus before they attend university may significantly benefit them. Yet if students are taught calculus before they are ready for the level of abstraction involved, they may abandon the study of mathematics altogether. So if we are going to teach pre-university students calculus, we must make sure they can handle the level of abstraction involved. Which one of the following principles most helps to justify the mathematics teacher's argument? ['Teachers who teach university-level mathematics to pre-university students should be aware that students are likely to learn effectively only when the application of mathematics to concrete problems is shown.', 'Cognitive tasks that require exceptional effort tend to undermine the motivation of those who attempt them.', 'Only those who, without losing motivation, can meet the cognitive challenges that new intellectual work involves should be introduced to it.', 'Only those parts of university-level mathematics that are the most concrete should be taught to pre-university students.']