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 `alpha` 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`
