Which statement correctly describes in-phase waves?

• A. Peaks and troughs occur at the same time and at the same location.
• B. Peaks and troughs occur at different times.
• C. The resultant wave is always smaller.
• D. They have opposite frequencies.

Answer: (A)

In-phase waves share zero phase difference, so their peaks and troughs line up at the same times and at the same locations. Because their displacements add together, they reinforce each other, yielding a larger resultant amplitude—a constructive interference effect. If peaks and troughs occurred at different times, there would be a phase difference and the waves would not reinforce each other as strongly. The statement about the resultant being always smaller is not true for in-phase waves, since they increase amplitude rather than decrease it. Opposite frequencies would prevent a constant in-phase relationship, since the waves would drift out of step over time.