\textbf{171.} To build a capacitor, we have two metal plates, one glass sheet (thickness $0.3\,\text{mm}$, $k=7$), one paraffin layer (thickness $0.1\,\text{cm}$, $k=2$), and one plastic layer (thickness $0.2\,\text{cm}$, $k=5$), and one layer with thickness $0.2\,\text{mm}$ ($k=3$). To maximize the capacitance, which material should we place between the metal plates?
(1) mica \hfill (2) glass \hfill (3) paraffin \hfill (4) plastic