When students do not find their assignments challenging, they become bored and so achieve less than their abilities would allow. On the other hand, when students find their assignments too difficult, they give up and so again achieve less than what they are capable of achieving. It is, ttherefore, clear that no student' s full potential will ever be realized. Which one of the following is an error of reasoning contained in the argument? ['Assuming without warrant that a situation allows only two possibilities.', 'Mistakenly equating what is actual and what is merely possible.', 'Confusing the coincidence of two events with a causal relation between the two.', 'Depending on the ambiguous use of a key term.']