Which of the following describes the difference between $x$ and $\Delta x$ ? $x$ represents a fixed quantity and $\Delta x$ represents a varying quantity. $x$ can be either positive or negative but $\Delta x$ must be positive. $x$ represents a quantity's value and $\Delta x$ represents the change in the quantity's value. $x$ represents a varying quantity and $\Delta x$ represents a fixed quantity. None of the above.

Step 1 ：The question is asking about the difference between $x$ and $\Delta x$. In mathematics, $x$ is often used to represent a variable or a specific value. On the other hand, $\Delta x$ is used to represent a change in the value of $x$. Therefore, the statement "x represents a quantity's value and $\Delta x$ represents the change in the quantity's value" seems to be the most accurate description.

Step 2 ：Final Answer: \(\boxed{x \text{ represents a quantity's value and } \Delta x \text{ represents the change in the quantity's value}}\)