A convex lens of focal length `f_1 ` and a concave lens of focal length `f_2` are placed in contact. The focal length of the combination is f. Then

Options

A. `f= f_1 - f_2`
B. `f = (frac(f_1 + f_2)(2))`
C. `(f_1f_2/f_1+f_2)`
D. `(f_1f_2 // f_2-f_1)`