A car's motion is represented by a distance-time graph. Describe the motion in segments OA, AB, and BC, and calculate the average speed for the first 10 seconds.

From the graph, at $t = 0\,\text{s}$, distance $= 0\,\text{m}$.
At $t = 10\,\text{s}$, distance $= 50\,\text{m}$.
Total distance $= 50\,\text{m} - 0\,\text{m} = 50\,\text{m}$.

Total time $= 10\,\text{s} - 0\,\text{s} = 10\,\text{s}$.

Average speed $= \frac{\text{Total distance}}{\text{Total time}}$

The car's average speed during the first 10s is $\boxed{\text{5 m/s}}$.