6. Firing a piece of pottery in a kiln takes place at different temperatures for different amounts of time. The graph below shows the temperatures in a kiln while firing a piece of pottery after the kiln is preheated to 200°F. During which time interval did the temperature in the kiln show the greatest average rate of change?a. 0 to 1 hourb. 1 hour to 1.5 hoursC. 2.5 hours to 5 hoursd. 5 hours to 8 hours