Two microwave sources $P$ and $Q$ emit coherent waves with a phase difference of $180^{\circ}$. These waves share the same wavelength $\lambda$. At point $S$, the interference pattern formed by the two sources has a minimum. The distance $(QS - PS)$ is known as the path difference. In the expressions given, $n$ is an integer. Which expression gives the path difference?
- A$n\lambda$
- B$\tfrac{1}{2}n\lambda$
- C$(n + \tfrac{1}{2})\lambda$
- D$(2n + \tfrac{1}{2})\lambda$