The ray diagram for two lenses kept at some distance is given in the figure. Which of the following options is correct? ($f_1, f_2 =$ focal lengths,$d =$ distance between lenses)

$A$ convex lens of focal length $f$ is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

$A$ beam of parallel rays is brought to a focus by a plano-convex lens. $A$ thin concave lens of the same focal length is joined to the first lens. The effect of this is:

$A$ bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. The refractive index $n$ of the first lens is $1.5$ and that of the second lens is $1.2$. Both the curved surfaces have the same radius of curvature $R = 14 \ cm$. For this bi-convex lens,if the object distance is $40 \ cm$,what will be the image distance (in $cm$)?

$A$ convex lens of focal length $30\, cm$ and a concave lens of focal length $10\, cm$ are placed coaxially at a distance $d$ apart. Parallel rays are incident on the convex lens. The rays emerging from the concave lens are also parallel. Then $d = $ . . . . . . $cm$.

$A$ parallel beam of light is incident on a system of two convex lenses of focal lengths $f_1 = 20\, cm$ and $f_2 = 10\, cm$. What should be the distance between the two lenses so that the rays after refraction from both the lenses emerge as a parallel beam (in $, cm$)? (Note: The condition for the final rays to emerge parallel is that the focal point of the first lens must coincide with the focal point of the second lens).