Answer: Remember that warmer air has a lower density and therefore lower pressure than cooler air. Remember also what happened to the ball above the hair dryer when you turned the heat on. If the ball was a plane, it would not end well for the plane.
When the air is hot as it was in Phoenix, Arizona the air pressure is already really low. In such conditions the planes are unable to get to a fast enough speed to generate enough of a difference in air pressure between the top and bottom of the wing and therefore generate sufficient lift to get the plane off the ground. The answer therefore is 1), The air was too hot for the planes to take off.
