Answer:
Let's answer the questions based on the "Altitude vs. Temperature" graph:
1. **Average temperature of the troposphere at earth's surface**: According to the graph, the average temperature of the troposphere at the earth's surface (0 km altitude) is approximately **15°C**.
2. **Average temperature of tree line area in the Rocky Mountains, Colorado**: The tree line in the Rocky Mountains of Colorado is above 11500 feet, which is approximately 3.5 km. Looking at the graph, the average temperature at this altitude is approximately **-20°C**.
3. **Altitude where the troposphere ends**: The troposphere ends where the temperature stops decreasing with altitude. According to the graph, this occurs at an altitude of approximately **15 km**.
