The displacement ' $x$ ' and time of travel 't' for a particle moving an a straight line are related as $t=$ $\sqrt{(x+1)(x-1)}$. Its acceleration at a time $t$ is

(A) $\frac{1}{x}-\frac{1}{x^{2}}$

(B) $\frac{1}{x^{3}}$

(C) $\frac{-1^{2}}{x^{3}}$

(D) $\frac{-1}{x^{2}}$