A block of mass 'm' is placed on a frictionless inclined plane of inclination $\mathrm{\theta }$ with horizontal. The inclined plane is accelerated horizontally so that the block does not slide down. In this situation, vertical force exerted by the inclined plane on the block is:

• A

  mg sin $\mathrm{\theta }$

• B

  mg cos $\mathrm{\theta }$

• C

  mg/cos $\mathrm{\theta }$

• D

  none of these