$35 \cdot 42$ If the sides of an equilateral triangle are divided into five equal parts, and lines parallel to the other two sides are drawn through each division point, dividing the equilateral triangle into a network of smaller equilateral triangles. Then the number of equilateral triangles of different sizes in the figure is 
(A) 48.
(B) 25.
(C) 45.
(D) 55.
(Shenyang City, Liaoning Province, China "Yucai Cup" Junior High School Mathematics Competition, 1988)
[Solution] Let $A B=A C=B C=5$, then the number of equilateral triangles with side length 1 in the figure is $1 + 3 + 5 + 7 + 9 = 25$;
the number of equilateral triangles with side length 2 is $1 + 2 + 3 + 4 + 3 = 13$;
the number of equilateral triangles with side length 3 is $1 + 2 + 3 = 6$;
the number of equilateral triangles with side length 4 is $1 + 2 = 3$;

the number of equilateral triangles with side length 5 is 1.
In total, there are various equilateral triangles
$$
25 + 13 + 6 + 3 + 1 = 48 \text{.}
$$

Therefore, the answer is $(A)$.