In the experimental setup shown in the figure above, a thin lens stands between a candle and a screen, A student moves the candle to several positions relative to the lens and the adjusts the position of the screen until an image is formed. Each time, the student records the distance `d` from the candle to the lens and the distance `s` from the lens to the screen. Which of the following procedure will allow the student to determine the focal length of the lens? `d/s = A/B =f/(s-f)` `d/s =f/(s-f)` `d(s-f) = sf` `ds-df = sf` `ds = df +sf` `1/f = (d + s)/(ds)` `1/f = 1/s + 1/d`