Wave speed = frequency × wavelength

Steps:

• Frequency f = 12 wavelengths produced in 1 s = 12 Hz (vibrator oscillation rate).
• Distance between first and third crest spans 2 wavelengths (one between first-second, one between second-third), so 2λ = 6 cm, λ = 3 cm.
• Wave speed v = f × λ = 12 × 3 = 36 cm/s.

Why C is correct:

• v = fλ directly applies the wave speed formula using the given production rate and crest spacing to yield 36 cm/s.

Why the others are wrong:

• A: Assumes f = 8 Hz (e.g., misreading 12 as 8), giving 8 × 3 = 24 cm/s.
• B: Arises from rough approximation (e.g., f ≈ 10 Hz, λ ≈ 2.5 cm), not matching data.
• D: Treats 6 cm as 1λ instead of 2λ, so λ = 6 cm, v = 12 × 6 = 72 cm/s.

Final answer: C