23. How many three-digit numbers have an odd number of factors?
A 5
B 10
C 20
D 21
E 22
23. E It can be shown that a positive integer has an odd number of factors if and only if it is square. The smallest three-digit square number is $10^{2}=100$ and the largest is $31^{2}=961$. Hence there are $31-9=22$ three-digit numbers which have an odd number of factors.