17. What odd digit can the sum of two prime numbers end with, if it is not a single-digit number?
17. If the sum of two numbers ends in an odd digit, then one addend is even, and the other is odd. Since among even numbers, the only prime number is 2, one of the addends must be 2. If the sum ends in the digit 1, then the second addend must end in the digit 9. Such a case is possible, for example, \(19+2=21\). If the sum ends in the digit 3, then the second addend must end in the digit 1. Such a case is also possible, for example, \(2+11=13\). If the sum ends in the digit 5, then the second addend must end in the digit 3. Such a case is possible, for example, \(2+13=15\). If the sum ends in the digit 7, then the second addend must end in the digit 5. Among two-digit numbers ending in the digit 5, there are no primes, so the sum cannot end in the digit 7. If the sum ends in the digit 9, then the second addend must end in the digit 7. Such a case is possible, for example, \(2+17=19\). Therefore, the sum can end in the digit \(1, 3, 5\) or 9.