With respect to a rectangular cartesian coordinate system, three vectors are expressed as
a = 4i-j
b = -3i +2j
c = -k
where i, j, k are unit vectors, along the x, y and z-axis respectively. The unit vector r along the direction of sum of these vectors is

• A

  $\mathrm{r}=\frac{1}{\sqrt{3}}(\mathrm{i}+\mathrm{j}-\mathrm{k})$

• B

  $\mathrm{r}=\frac{1}{\sqrt{2}}(\mathrm{i}+\mathrm{j}-\mathrm{k})$

• C

  $\mathrm{r}=\frac{1}{3}(\mathrm{i}-\mathrm{j}+\mathrm{k})$

• D

  $\mathrm{r}=\frac{1}{\sqrt{2}}(\mathrm{i}+\mathrm{j}+\mathrm{k})$