Let $r$ and $\theta$ be the polar coordinates defined by $x=r \cos \theta$ and $y=r \sin \theta$. The area of the cardioid $r=a(1-\cos \theta), 0 \leq \theta \leq 2 \pi$, is
- $\frac{3 \pi a^{2}}{2}$
- $\frac{2 \pi a^{2}}{3}$
- $3 \pi a^{2}$
- $2 \pi a^{2}$