Electromagnetism - Oscilloscope

The figure above represents the trace on the screen of cathode ray oscilloscope. The screen is graduated in centimeters. The spot on the screen moves horizontally with a constant speed of 0.5 centimeter/millisecond and the vertical scale is 2 volts/centimeter. The signal is a superposition of two oscillations. Which of the following are most nearly the observed amplitude and frequency of these two oscillations? 

Oscillation 1 Oscillation 2 A. 5V, 250Hz 2.5V, 1000Hz B.  1.5V, 250Hz 3V, 1500Hz  C. 5V, 6Hz 2V, 2Hz  D. 2.5V, 83Hz 1.25V, 500Hz  E. 6.14V, 98Hz                1.35V, 257Hz

(GR9677 #28)

Solution:

The graph shows one big λ consists of 6 small λs.
λ_big ≈ 6 cm
λ_small ≈ 1 cm 

v = λf

Given: v = 0.5 cm/ms
f_big = v/λ_b = 0.5 cm/(6 cm ms) = 1/ (12 ms) = 10³/(12 s) = 83 Hz (Osc. 1)
f_small = v/λ_s = 0.5 cm/(1 cm ms) = 1/ (2 ms) = 10³/(2 s) = 500 Hz (Osc. 2)

Answer: D