30. As shown in the figure, $A D, B E, C F$ are the three medians of $\triangle A B C$, and the three medians intersect at point $O$. Then, the number of pairs of triangles with equal area in the figure is ( ).
(A) 15.
(B) 18.
(C) 30.
(D) 33.
Answer: D.
Solution: Let the area of $\triangle ABC$ be $S$, then
the figure contains 6 triangles with an area of $\frac{S}{6}$, which can form $\frac{6 \times 5}{2}=15$ pairs of triangles with an area of $\frac{S}{6}$; the figure contains 3 triangles with an area of $\frac{S}{3}$, which can form $\frac{3 \times 2}{2}=3$ pairs of triangles with an area of $\frac{S}{3}$; the figure contains 6 triangles with an area of $\frac{S}{2}$, which can form $\frac{6 \times 5}{2}=15$ pairs of triangles with an area of $\frac{S}{2}$.

Therefore, the figure contains a total of $15+3+15=33$ pairs of triangles with equal areas.
Hence, the answer is D.