In the figure, an air lens of radii of curvature 10 $\mathrm{cm}\left({\mathrm{R}}_1={\mathrm{R}}_2=10\mathrm{cm}\right)$ is cut in a cylinder of glass,$(\mathrm{\mu }=1.5)$. The focal length and the nature of the lens is

• A

  15 cm, concave

• B

  15 cm, convex

• C

  $\mathrm{\infty },\text{ neither concave nor convex }$

• D

  $0, \mathrm{concave}$