If the normals at the points $\mathrm{P}, \mathrm{Q}, \mathrm{R}$ on the parabola $\mathrm{y}^{2}=4 \mathrm{ax}$ intersect at the point $(\alpha, \beta)$, then the orthocentre of triangle $\mathrm{PQR}$ is
(a) $\left(\frac{2}{3} \mathrm{a}, 0\right)$
(b) $\left(\alpha-3 \mathrm{a}, \frac{-\beta}{2}\right)$
(c) $\left(\alpha-6 \mathrm{a}, \frac{-\beta}{2}\right)$
(d) $\left(\frac{\alpha}{2}, \frac{\beta}{2}\right)$