Question id 312 - Preflight Preparation - FAA PPL(A) - Private Pilot Airplane

Altitude has no effect on the fuel/air mixture in modern airplane engines.
The fuel-air mixture is leaner because the density of air increases while the amount of fuel remains constant.
The fuel-air mixture becomes richer because the density of air decreases while the amount of fuel remains constant

Training tests and timed exam simulations FAA PPL(A) - Private Pilot Airplane