An insect crawls up a hemispherical surface very slowly (see the figure). The coefficient of friction between the insect and the surface is 1/3. If the line joining the centre of the hemispherical surface to the insect makes an angle ex with the vertical, the maximum possible value of $\alpha$ is given by

 

• A

  $\cot \alpha =3$

• B

  $\tan \alpha =3$

• C

  $\sec \alpha =3$

• D

  $cosec\alpha =3$