Assertion (A) :  $\overline{\mathrm{a}},\overline{\mathrm{b}},\overline{\mathrm{c}}$  and $\overline{\mathrm{d}}$ are the position vectors of four coplanar points A, B, C, D. If $\left|\overline{\mathrm{d}}-\overline{\mathrm{a}}\right| = \left|\overline{\mathrm{d}}-\overline{\mathrm{b}}\right| = \left|\overline{\mathrm{d}}-\overline{\mathrm{c}}\right|,$ hen D is the circumcentre of $∆$ABC

Reason (R) : In any triangle, the circumcentre is equidistant from the vertices. Then