7. (1984 National High School League Question) For all $m, n$ satisfying $1 \leqslant n \leqslant m \leqslant 5$, the number of different hyperbolas represented by the polar equation $\rho=$ $\frac{1}{1-C_{m}^{n} \cos \theta}$ is ( ).
A. 5
B. 10
C. 7
D. 6
7. D. The necessary and sufficient condition for the equation to represent a hyperbola is $e=C_{m}^{n}>1$, and $1 \leqslant m \leqslant 5$. There are 6 different values of $C_{m}^{n}>1$, namely $C_{2}^{1}, C_{3}^{1}, C_{4}^{1}, C_{1}^{2}, C_{5}^{1}, C_{5}^{2}$, which correspond to 6 different hyperbolas. Therefore, the answer is D.