5. Divide each side of an equilateral triangle into four equal parts, and then draw lines through these points parallel to the other two sides. Then the number of rhombuses formed by the line segments in Figure 1 is ( ).
(A) 15
(B) 18
(C) 21
(D) 24
5.C.

In Figure 1, there are only two types of rhombi with side lengths of 1 or 2. Each rhombus has exactly one diagonal equal to its side length. Inside the original equilateral triangle, each line segment of length 1 is exactly the diagonal of a rhombus with side length 1; there are 18 such line segments, corresponding to 18 rhombi with side length 1. Each median of the original equilateral triangle is exactly the diagonal of a rhombus with side length 2, and the three medians correspond to 3 rhombi with side length 2. In total, there are 21 rhombi.