A convergent doublet of separated lenses, corrected for spherical aberration, has resultant focal length of 10 cm . The separation between the two lenses is 2 cm . The focal lengths of the component lenses are:\\
(1) $18 \mathrm {~cm} , 20 \mathrm {~cm}$\\
(2) $10 \mathrm {~cm} , 12 \mathrm {~cm}$\\
(3) $12 \mathrm {~cm} , 14 \mathrm {~cm}$\\
(4) $16 \mathrm {~cm} , 18 \mathrm {~cm}$