Given that -10 represents 10 seconds before a runner finishes his first lap and 10 represents 10 seconds after the runner finishes his first lap then zero will represent;
When the runner hits the -10 mark, it will require the runner to run an extra 10 seconds to reach the finishing point for lap one. Mathematically,
-10 + 10 = 0 ------the runner will be at the zero mark of the lap, so
zero will be attained after 10 seconds from -10, to mean that at the zero mark from the -10 mark, the runner took ten seconds.
Answer;
C. The time when the runner finishes his first lap