To solve this problem, we need to determine which line in the graph has a zero gradient.

• A line with a zero gradient is horizontal because the slope (or gradient) is the change in $y$ over the change in $x$, and for a horizontal line, the $y$-coordinate stays constant while the $x$-coordinate changes.

Looking at the provided graph:

• Line C (the blue line) is horizontal, running along the $x$-axis. Since it has no change in the $y$-coordinate as $x$ changes, it has a zero gradient.

Final Answer:

• The line with a zero gradient is Line C.

Would you like further details on gradients or have any questions?

Here are 5 related questions to explore this concept further:

1. What does it mean when a line has a positive gradient?
2. How can you calculate the gradient of a line given two points?
3. Which line in the graph has a negative gradient?
4. What is the gradient of a vertical line, and how is it different from a horizontal line?
5. How does the steepness of a line relate to its gradient value?

Tip: The gradient of a line is the measure of its steepness; horizontal lines have a gradient of zero, while vertical lines have an undefined gradient.