If $\mathrm{tan\theta }\mathrm{\hspace{0.33em}}=\mathrm{\hspace{0.33em}}\frac{1}{\sqrt{5}}$ and $\mathrm{\theta }$ lies in the first quadrant, the value of cos $\mathrm{\theta }$ is :

• A

  $\sqrt{\frac{5}{6}}$

• B

  $-\sqrt{\frac{5}{6}}$

• C

  $\frac{1}{\sqrt{6}}$

• D

  $-\frac{1}{\sqrt{6}}$