An alternating current is given by the equation $i = i_1 \cos \omega t + i_2 \sin \omega t$. The r.m.s. current is given by

• A
  $\frac{1}{\sqrt{2}}(i_1 + i_2)$
• B
  $\frac{1}{\sqrt{2}}(i_1 + i_2)^2$
• C
  $\frac{1}{\sqrt{2}}(i_1^2 + i_2^2)^{1/2}$
• D
  $\frac{1}{2}(i_1^2 + i_2^2)^{1/2}$