Domain of the function $y=\sqrt{\frac{1-5^x}{7^{-x}-7}}$ is
(1) R                                                 (2) (–∞, –1) ∪ [0, + ∞)
(3) (–∞, –1] ∪ [0, + ∞)                     (4) (–∞, –1] ∪ (0, + ∞)