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A convex lens produces an image of a candle flame upon a screen whose distance from candle is \( D \). When the lens is displaced through a distance \( x \), (the distance between the candle and the screen is kept constant), it is found that a sharp image is again produced upon the screen. Find the focal length of the lens in terms of \( D \) and \( x \).
(a) \( f=\left(D^{2}-x^{2}\right) / 4 D \)
(b) \( f=\left(D^{2}-x^{2}\right) / 3 D \)
(c) \( f=\left(D^{2}-x^{2}\right) / 7 D \)
(d) \( f=\left(D^{2}-x^{2}\right) / 2 D \)
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