The regression analysis below relates US annual energy consumption in trillions of BTUs to the independent variables "US Gross Domestic Product (GDP) in trillions of dollars" and "average gas mileage of all passenger cars in miles per gallon (mpg)."

The coefficient for the independent variable "average car gas mileage (mpg)," -70.50, describes:

1. The relationship between energy consumption and average car gas mileage, controlling for GDP.
2. The relationship between energy consumption and average car gas mileage, not controlling for GDP.
3. The relationship between average car gas mileage and GDP, controlling for energy consumption.
4. The relationship between average car gas mileage and GDP, not controlling for energy consumption.