A student read that putting an extra 5 psi (pounds per square inch) of air into car tires helps it burn less fuel, so she decides to test this idea. She brings her car to a race track, fills the gas tank, and drives 100 miles. She then measures how much fuel she burned. She then adds another 5 psi of air to the tires, drives another 100
miles, and measures the fuel burned. She then compares the results.

What is the controlled variable:
1. The weather conditions during each test of 100 miles
2. The speed the car is driven on
3. The race track that the car is driven on
4. All of the above

Explain: