Which one of the following options is the correct Fourier series of the periodic function \( f(x) \) described below:
\[
f(x) =
\begin{cases}
0, & \text{if } -2 < x < -1, \\
2k, & \text{if } -1 \leq x < 1, \quad \text{period} = 4, \\
0, & \text{if } 1 \leq x < 2.
\end{cases}
\]