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The magnifying power of telescope is 9 . When it is adjusted for parallel rays, the distance between the objective and eyepiece is 20 cm. The focal lengths of the lenses are
Difficulty: Easy
A: 
$\rm{10\, cm,\,10\,cm}$
B: 
$\rm{15\, cm,\,5\,cm}$
C: 
$\rm{18\, cm,\,2\,cm}$
D: 
$\rm{11\, cm,\,9\,cm}$