Two sinusoidal waveforms are shown in the diagram. At time $t = 0$ the waves are in phase. The dotted line marks $t = 18\,\text{s}$. At which time is the phase difference between the two oscillations $\frac{1}{8}$ of a cycle?
- A$4.0\,\text{s}$
- B$4.5\,\text{s}$
- C$8.0\,\text{s}$
- D$9.0\,\text{s}$