In the diagram, two wave sources, $S_1$ and $S_2$, are shown. Their oscillations have a phase difference of $180^{\circ}$. Each source produces a wave with wavelength $2.0\,\text{cm}$. By the time the wave from each source reaches point $P$, its amplitude is $x_0$. What is the amplitude of the oscillation at $P$?
- A$0$
- B$\frac{x_0}{2}$
- C$x_0$
- D$2x_0$