If three different natural numbers are given, the sum of two of them must be even. Why?
For three different natural numbers, there are only the following cases: ① three odd numbers, then the sum of any two must be even; ② three even numbers, the sum of any two must be even; ③ two odd numbers, one even number, the sum of two odd numbers is even; ④ two even numbers, one odd number, the sum of two even numbers is even The sum of two numbers must be even