Choice A is the correct answer.

1. Formula: $\Delta = b^2 - 4ac$.
2. If $a$ and $c$ have opposite signs, their product $ac$ is negative.
3. Therefore, $-4ac$ becomes a positive number (subtracting a negative).
4. Since $b^2 \geq 0$, adding a positive number guarantees the result is positive.
5. Thus, there are always two real roots.

Choice B is incorrect. Choice C is incorrect. Choice D is incorrect.