A thin uniform disc (see figure) of mass M has outer radius 4R and inner radius 3R. The work required to take a unit mass for point P on its axis to infinity is
A. `(2GM)/(7R)(4sqrt(2)-5)`
B. `-(2GM)/(7R)(4sqrt(2)-5)`
C. `(GM)/(4R)`
D. `(2GM)/(5R) (sqrt(2)-1)`
