[单选题]
Two thin lenses, of ff0: ; xhxke(mxhota .)f7wakwn-,s9e,cf .tehh 2dn4ocal 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. $f_1 f_2/(f_1 + f_2)$.
