1. If all the diagonals of a convex $n$-sided polygon $F (n \geqslant 4)$ are equal, then
(A) $F \in\{$ quadrilateral $\}$;
(B) $F \in\{$ pentagon $\}$;
(C) $F \in\{$ quadrilateral $\} \cup\{$ pentagon $\}$;
(D) $F \in\{$ equilateral polygon $\} \cup\{$ equiangular polygon $\}$.
Answer ( )