Two convex lenses of focal lengths `f_(1) and f_2` are separated co-axially by a distance d. The power of the combination will be one if

Two convex lenses of focal length f_1 and f_2 are mounted coaxially separated by a distance. If the power of the combination is zero, the distance between the lenses is

Two convex lenses of focal length f_1 and f_2 are mounted coaxially separated by a distance. If the power of the combination is zero, the distance between the lenses is

Two thin convex lenses of focal lengths f_(1) and f_(2) are in contact and coaxial. The power of the combination is

Two lenses of focal legths f_(1) and f_(2) are kept in contact coaxially. The resultant power of combination willl be

Two thin convex lenses of focal lengths f_1 and f_2 are placed at a distance d between them. For the power of combination to be zero, the separation d should fee :

Two thin lenses of focal length f_(1) and f_(2) are in contact and coaxial. The power of the combination is