3. Which of the following has exactly one factor other than 1 and itself?
A 6
B 8
C 13
D 19
E 25
Solution: $\mathbf{E}$
The factors of 6 are $1,2,3$ and 6 ; the factors of 8 are $1,2,4$ and 8 ; the factors of 13 are 1,13 ; the factors of 19 are 1,19 ; and the factors of 25 are $1,5,25$. We see from this that, of the numbers we are given as options, only 25 has exactly one factor other than 1 and itself.