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A student focused the image of a distant object using a device $'X\ ’$ on a while screen $'S\ ’$ as shown in the figure. If the distance of the screen from the device is $40 \ cm,$ select the correct statement about the device.

• A
  The device $X$ is a convex lens of focal length $20 \ cm.$
• B
  The device $X$ is a concave mirror of focal length $40 \ cm.$
• C
  The device $X$ is a convex mirror of radius of curvature $40 \ cm.$
• ✓
  The device $X$ is a convex lens of focal length $40 \ cm.$