A student sends parallel beams of light through four different converging lenses. For each trial, he measures the distance $x$ and the distance $y$. Which lens has the greatest focal length?
- A$x = 4.6\,\text{cm},\; y = 2.0\,\text{cm}$
- B$x = 5.1\,\text{cm},\; y = 3.1\,\text{cm}$
- C$x = 5.9\,\text{cm},\; y = 2.3\,\text{cm}$
- D$x = 6.1\,\text{cm},\; y = 2.4\,\text{cm}$