An ac supply gives 30 V r.m.s. which passes through a 10$\mathrm{\Omega }$ resistance. The power dissipated in it is 

• A

  $90\sqrt{2}\mathrm{W}$

• B

  $90\mathrm{W}$

• C

  $45\sqrt{2}\mathrm{W}$

• D

  $45\mathrm{W}$