Given a real number $a>0$, consider the triangle $\Delta$ with vertices $0, a, a+i a .$ If $\Delta$ is given the counter clockwise orientation, then the contour integral $\oint_{\Delta} \operatorname{Re}(z) d z$ (with $\operatorname{Re}(z)$ denoting the real part of $z$ ) is equal to
2. $i \frac{a^{2}}{2}$
3. $i a^{2}$
4. $i \frac{3 a^{2}}{2}$