$18 \cdot 26$ In a heptagram, the number of convex quadrilaterals is
(A) 7.
(B) 8.
(C) 9.
(D) 10.
(Shanghai Junior High School Mathematics Competition, 1982)
[Solution] Analyze each side one by one:
For the quadrilateral with $AB$ as one side, there are two convex quadrilaterals: $ABLG$ and $ABCP$.
Similarly, for the other 6 sides, the same result applies.
Note that each convex quadrilateral is counted twice in the above calculation, so there are
7 convex quadrilaterals in the figure.
Therefore, the answer is $(A)$.