A block is kept on a frictionless inclined surface with angle of inclination a . The incline is given an acceleration ‘a’ to keep the block stationary. Then ‘a’ is equal to

• A

  $\frac{\mathrm{g}}{\tan \mathrm{\alpha }}$

• B

  $\mathrm{gcosec}\mathrm{\alpha }$

• C

  g

• D

  $\mathrm{gtan}\mathrm{\alpha }$