Which of the following is equivalent to x/x^2-1 + 4/x^2-6x-7?
a. x(x-7)+4(x-1)/(x+1)^2(x-7)
b. x+4/(x+1)^2(x-7)
c. x(x-7)+4(x-1)/(x+1)(x-1)(x-7)
d. x+4/(x+1)(x-1)(x-7)