Let the roots of the equation `(3x^3-x^2+x-1)/(3x^3-x^2-x+1)=(4x^3+7x^2+x+1)/(4x^3+7x^2-x-1)` be `x_1`,`x_2` and `x_3` , then the value of `x_1+x_2+x_3` is
a.   `0`
b.   `1`
c.   `-1`
d.   None of these