Two thin convex lenses of focal length $f_1$ and $f_2$ are placed in contact with each other. The equivalent power of the lens combination is . . . . . . .
- A$f_1 \times f_2$
- B$\frac{f_1+f_2}{f_1 \times f_2}$
- C$f_1 + f_2$
- D$\frac{f_1 \times f_2}{f_1 + f_2}$
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