3. (1990 Fujian Province High School Competition Question) The natural number values of $n$ that make $n^{5}-5 n^{3}+4 n+7$ a perfect square ( ).
A. There is exactly one
B. There are a finite number, but more than one
C. There are infinitely many
D. None exist
3. D. It is easy to know that $n^{5}-5 n^{3}+4 n+7=(n-2)(n-1) n(n+1)(n+2)+7 \equiv 7(\bmod 10)$, and an integer with a units digit of 7 is not a perfect square. Therefore, the answer is D.