A straight rod of length `L` extends from `x=a` to `x=L+a`. Find the gravitational force exerts on a point mass `m` at `x=0` is (if the linear density of rod`mu=A+Bx^(2)`)
A. `Gm(a(1/alpha-1/(alpha+l))+bl)`
B. `(Gm(a+bx^2))/l^2`
C. `Gm(alpha(1/a-1/(a+l)+bl)`
D. `Gm(a(1/(alpha+l)-1/alpha))+bl)`