$\mathrm{A}\left(\mathrm{z}_{1}\right), \mathrm{B}\left(\mathrm{iz}_{1}\right)$ form a triangle with the origin. For $\triangle \mathrm{OAB}$,
(a) circumcentre is $\frac{z_{1}(1-i)}{2}$
(b) circumcentre is $\frac{z_{1}(1+i)}{2}$
(c) orthocenter is at the origin
(d) the altitudes through $\mathrm{O}, \mathrm{A}$ and $\mathrm{B}$ are in the ratio $1: \sqrt{2}: \sqrt{2}$