13. How many sets of three consecutive integers are there in which the sum of the three integers equals their product?
A 0
B 2
C 3
D 4
E 5
13. C Let the three consecutive integers be $n-1, n$ and $n+1$. Then $(n-1) n(n+1)=n-1+n+n+1$. So $n\left(n^{2}-1\right)=3 n$. Hence $n\left(n^{2}-4\right)=0$. Therefore $n=0$ or $n^{2}=4$. So $n=-2,0$ or 2 and the sets of three consecutive integers in which the sum of the integers equals their product are $\{-3,-2,-1\},\{-1,0,1\}$ and $\{1,2,3\}$.