[单选题]
Two thin lenses, of fo5bs8 8quf.cnwq od wo(s ojqs0(gndv.i3j 806c hoyrob) uy1pbhe5jq.61t 3d)al lengths $f_1$ and $f_2$ placed in contact with each other are equivalent to a single lens of focal length of
A. $f_1 + f_2$.
B. $1/(f_1 + f_2)$.
C. $(f_1 + f_2)/f_1 f_2$.
D. $f_1 f_2/(f_1 + f_2)$.
