9. How many positive integers $N$ possess the property that exactly one of the numbers $N$ and $(N+20)$ is a 4-digit number?
A 19
B 20
C 38
D 39
E 40
9. E If exactly one of $N$ and $N+20$ has four digits, then the other has either 3 or 5 digits. If $N$ has 3 digits and $N+20$ has 4 digits, then $980 \leqslant N \leqslant 999$, giving 20 possibilities. If $N$ has 4 digits and $N+20$ has 5 digits, then $9980 \leqslant N \leqslant 9999$, giving 20 possibilities. Overall there are 40 possibilities for $N$.