A fitness coach recorded the time x (in minutes) that each runner trained on a certain day and the runner’s resting heart rate y (in beats per minute) the next morning. The scatterplot shows the data, and a line of best fit is drawn.
Which choice is the average rate of change, in beats per minute per minute of training, of the predicted resting heart rate from x=20 to x=50 according to the line of best fit?