Two thin lenses of focal lengths $f_1$ and $f_2$are in contact , the equivalent focal length is

Question

$(a)\;f_1+f_2 \\ (b)\;f_1 \times f_2 \\ (c)\;\frac{f_1f_2}{f_1 +f_2} \\ (d)\;f_1 -f_2 $