Find the transformer ratios $a$ and $b$ such that the impedance ($Z_{in}$) is resistive and equals $2.5 \Omega$ when the network is excited with a sine wave voltage of angular frequency of $5000$ rad/s.

- $a = 0.5, b = 2.0$
- $a = 2.0, b = 0.5$
- $a = 1.0, b = 1.0$
- $a = 4.0, b = 0.5$