7) Every year, at the time of paying taxes, the user makes a declaration regarding the current year. If the declaration is true, they must pay the taxes; if it is false, they do not pay. A young mathematician, who considers the system unfair, finds a way to block it with one of the following statements: which one?
(A) "Fish live in water"
(B) "I live in water"
(C) "Fish do not pay taxes"
(D) "I do not pay taxes"
(E) "I pay taxes".
7) The answer is (D). Obviously, the first three statements cannot cause problems. The fifth is more curious, but it allows the system to run smoothly, whether the mathematician pays or does not pay taxes. It is only the fourth that cannot work: indeed, if the mathematician pays the taxes, it becomes false, and thus he should not have paid them; conversely, if he does not pay them, then it is true and he should have paid them.