If,  $0.5\widehat{\mathrm{i}}+0.8\widehat{\mathrm{j}}+\mathrm{c}\widehat{\mathrm{k}}$ is a unit vector, then the value of c is

• A

  $\sqrt{0.11}$

• B

  $\sqrt{0.22}$

• C

  $\sqrt{0.33}$

• D

  $\sqrt{0.89}$