The complex numbers $z_{1}, z_{2}, z_{3}$ and $z_{4}$ represent the vertices of a parallelogram taken in order if and only if
(a) $\mathrm{z}_{1}+\mathrm{z}_{3}=\mathrm{z}_{2}+\mathrm{z}_{4}$
(b) $\mathrm{z}_{1}+\mathrm{z}_{2}+\mathrm{z}_{3}=\mathrm{z}_{4}$
(c) $\mathrm{z}_{1}+\mathrm{z}_{2}=\mathrm{z}_{3}+\mathrm{z}_{4}$
(d) $\mathrm{z}_{1}-\mathrm{z}_{2}=\mathrm{z}_{3}-\mathrm{z}_{4}$