Identify the time coordinates of the points on the wave.

Step 1: Identify the time coordinates of the points on the wave.
From the given displacement-time graph:

• The wave starts at $O$ ($t=0$).
• Point $P$ is at the first crest. A full period ($T$) is the time for one complete oscillation. Visually, one complete oscillation ends at point $C$. So, the time coordinate of $C$ is $T$.
• The first crest (point $P$) occurs at $t_P = \frac{T}{4}$.
• Point $B$ is where the wave crosses the time axis going downwards after the first crest. This occurs at $t_B = \frac{T}{2}$.
• Point $D$ is where the wave crosses the time