26. Among the numbers $2004, 2005, 2006, 2007$, the number that cannot be expressed as the difference of squares of two integers is ( ).
(A) 2004
(B) 2005
(C) 2006
(D) 2007
26. C.

From $x^{2}-y^{2}=(x+y)(x-y)$, where $x+y$ and $x-y$ have the same parity, we know that $x^{2}-y^{2}$ does not leave a remainder of 2 when divided by 4. Since 2006 leaves a remainder of 2 when divided by 4, 2006 cannot be expressed as the difference of squares of two integers.