As part of a study of lake conditions, the water temperature was recorded at different depths below the surface of a certain lake. The data shown in the scatterplot give the recorded temperature, in degrees Celsius, for $7$ depths, in meters, below the surface of the lake.

• The scatterplot has 7 data points.
• The data points are in a linear pattern trending down from left to right.
• The data points have the following approximate coordinates:
  
  • (15 comma 21.80)
  • (22 comma 20.70)
  • (35 comma 15.70)
  • (47 comma 16.80)
  • (51 comma 15.70)
  • (54 comma 13.70)
  • (70 comma 12.70)

Which of the following is closest to the slope of a line of best fit for the data shown?