Final answer:
The letter that indicates the most gradual change in temperature is Letter A, while the letter that indicates the most sudden change in temperature is Letter D. The average temperatures in the middle of the United States are warmer than the coasts mainly due to altitude.
Step-by-step explanation:
The letter that indicates the most gradual change in temperature is Letter A, while the letter that indicates the most sudden change in temperature is Letter D. The average temperatures in the middle of the United States are warmer than the coasts mainly due to altitude. As you move away from the coasts towards the middle of the country, the land gradually rises in elevation, leading to higher average temperatures.