Data Sufficiency

This question is from Princeton Review's Full Tests. I chose 'E' as the answer. But the official answer is B. The explanation provided is that since the square of y is always +v3, thus knowing the signs of x and z is sufficient to answer the question.

It seems like a silly question to ask.

I think the explanation is incorrect since the equation won't hold true for all values of x and z(<0).

Please correct me if I am making a mistake.

Sanjay