Assertion (A): $\cos^2 \alpha + \cos^2\left(\alpha + \frac{\pi}{3}\right) + \cos^2\left(\alpha + \frac{4\pi}{3}\right) = 3 \cos \alpha \cos\left(\alpha + \frac{2\pi}{3}\right) \cos\left(\alpha + \frac{4\pi}{3}\right)$.

Reason (R): Si $a + b + c = 0$, entonces $a^3 + b^3 + c^3 = 3abc$.

(a) A (b) B (c) C (d) D