Four identical metal plates, each of area $A$, are placed parallel to one another with a gap $d$ between neighbours, in air, as shown in the figure. Plates 1 and 3 are joined to terminal X, and plates 2 and 4 are joined to terminal Y. If $C = \dfrac{\varepsilon_0 A}{d}$, the capacitance between X and Y is

Fig. 1: Four parallel metal plates, 1 to 4, a distance d apart. Plates 1 and 3 are joined to terminal X, and plates 2 and 4 to terminal Y.