A network consisting of a finite number of linear resistor (R), inducer (L), and capacitor (C) elements, connected all in series or all in parallel, is excited with a source of the form
$$\sum\limits_{k = 1}^3 {{a_x}\,\cos \left( {k{\omega _0}t} \right),{\rm{were}}\,{a_k} \ne 0,} \,{\omega _0} \ne 0.$$
The source has nonzero impedance. Which one of the following is a possible form of the output measured across a resistor in the network?

A. $$\sum\limits_{k = 1}^3 {{b_x}\,\cos \left( {k{\omega _0}t + {\phi _k}} \right),{\rm{were}}\,{b_k} \ne {a_k},} \,\forall K$$

B. $$\sum\limits_{k = 1}^3 {{b_x}\,\cos \left( {k{\omega _0}t + {\phi _k}} \right),{\rm{were}}\,{b_k} \ne 0,} \,\forall K$$

C. $$\sum\limits_{k = 1}^3 {{a_x}\,\cos \left( {k{\omega _0}t + {\phi _k}} \right)} $$

D. $$\sum\limits_{k = 1}^2 {{a_x}\,\cos \left( {k{\omega _0}t + {\phi _k}} \right)} $$

Answer: Option C