A1. Samo wrote a 3-digit odd natural number on a piece of paper and told Peter only the last digit of this number. Peter immediately realized that the number Samo wrote on the paper is not a prime number. Which digit did Samo tell Peter?
(A) 1
(B) 3
(C) 5
(D) 7
(E) 9
A1. From the last digit of a natural number in the trimester, we can determine that it is not a prime number only if the last digit is even (and thus the number is divisible by 2) or if it is 5 (and thus the number is divisible by 5). All other digits can be the last digits of a trimester prime number, for example, the numbers 101, 103, 107, and 109 are all prime numbers. Since Sam's number was odd, its last digit was 5.