*6. If $m, n$ are non-negative integers not greater than 6, then $C_{6}^{m} x^{2}+C_{6}^{n} y^{2}=1$ represents ( ) different ellipses.
(A) $P_{7}^{2}$
(B) $P_{6}^{2}$
(C) $C_{4}^{2}$
(D) $P_{4}^{2}$.
6. D
$C_{6}^{k}$ has four values: $1,6,15,20$. The problem is transformed into the number of permutations of choosing 2 elements from 4 different elements, so there are $P_{4}^{2}$ different ellipses.