$26 \cdot 17$ For all $m, n$ satisfying $1 \leqslant n \leqslant m \leqslant 5$, the polar equation
$$
\rho=\frac{1}{1-C_{m}^{n} \cos \theta}
$$

represents the number of different hyperbolas is
(A) 15 .
(B) 10 .
(C) 7 .
(D) 6 .
(China High School Mathematics League, 1984)