In the displacement method, a convex lens is placed in between an object and a screen if the magnification in the two position are `m_1` and `m_2(m_1gtm_2)` and the distance between the two positions of the lens is x, the focal length of the lens is
A. `(x)/(m_1+m_2)`
B. `(x)/(m_1-m_2)`
C. `(x)/((m_1+m+2)^2)`
D. `(x)/((m_1-m_2)^2)`
