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Correct option is B)

The image of the object can be located on the screen for two-position of convex lens such that u and v are exchanged.

The separation between two positions of the lens is, d= 20 cm

From the figure,

$u_{1}+v_{1}=90cm$

$v_{1}โu_{1}=20cm$

Solving,

$v_{1}=55cm,ยu_{1}=35cm$

From lens formula,

$v1โโu1โ=f1โ$ย

$551โโโ351โ=f1โ$

$โf1โ=551โ+351โ=9055ร35โ$

$โf=21.4cm$

$โf=21.4cm$

Hence, the correct option is $(B)$

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