In the visual model of (a+b+c)^2, how many rectangular cross parts are there in total?

Correct answer: 6

The direct answer is D: there are 6 rectangular cross parts. Imagine a square whose side is split into a, b, and c. In the area model, the three diagonal square parts are a², b², and c². For each pair of different lengths, there are two rectangles: one pair gives ab and ba, another gives ac and ca, and the third gives bc and cb. Thus the rectangular cross parts are 2ab, 2ac, and 2bc when paired by type, but physically they are six separate rectangles: two ab, two ac, and two bc. Therefore (a+b+c)^2=a²+b²+c²+2ab+2ac+2bc. A, B, and C undercount these pairwise rectangles. Memory cue: with three side parts, choose three pairs, and each pair appears twice: 3×2=6.