The graph illustrates how the displacement of a particle in a progressive wave changes with time. Two readings, $x$ and $y$, are marked on the graph. What do $x$ and $y$ stand for?
- A$x$: amplitude, $y$: period
- B$x$: frequency, $y$: period
- C$x$: amplitude, $y$: wavelength
- D$x$: frequency, $y$: wavelength