Well... You're not entirely wrong, I'll tell you that, but when I said line AD = line DC, I meant in terms of length. While the two are not parallel but instead perpendicular, they are still congruent, thus proving that AC=DC. B's sole purpose, for sake of the joke, was to not make it seem like AB=CB, but instead make it say AC=DC. Thus, the reference to Back in Black.
See? I'm smart.