D, Each x value has one and only one y value.; This is because if an x-value repeats itself then it is not a function. You can test this on a graph by drawing a vertical line on it. If the vertical line intercepts the line more than once anywhere on the graph, then it is not a function.
Elite marathon race times improved from 5 to ~20 years, remained linear between ~20 and ~35 years, and started to increase at the age of ~35 years in a curvilinear manner with increasing age in both women and men. The sex difference in elite marathon race time increased non-linearly and was lowest at the age of ~49 years.
Ten thousands is the answer
