Undefined quantities on QC questions

GRE GRE Quant Problem Solving
1

What should I do on the GRE when I run into an undefined quantity on a quantitative comparison question?

Example: x<0

Quantity A: \frac{x-1}{x^2+1}

Quantity B: \frac{x^3-x}{x^4-1}

Quantity B is undefined at x=-1 but quantity A is not. Otherwise, quantity B is larger because it simplifies to \frac{x}{x^2+1} and since both quantities have the same denominator, the one with the larger numerator (quantity B) is larger. Would the answer to this problem be quantity B or “cannot determine"?

2

You cannot compare invalid values, so D.

3

Moreover, the GRE will never ask you something like this. The only way such a question would be asked is if they clarified that the two given quantities are indeed functions. At that point, the answer would be much clearer.