The position vector of the point $(4,5,6)$ is (a) $4 \vec{i}+5 \vec{j}+6 \vec{k}$ (b) $4 \vec{i}-5 \vec{j}-6 \vec{k}$ (c) $2 \vec{i}+\vec{j}+\vec{k}$ (d) $\vec{i}+\vec{j}+\vec{k}$