Final answer:
Avoid topping off the fuel tank because it can saturate the charcoal canister, impairing the vehicle's evaporative emission control system, which is designed to capture and recycle fuel vapors.
Step-by-step explanation:
The question is related to the practice of topping off the fuel tank during refueling and why it should be avoided. Topping off the tank can lead to several issues. When you fill the tank to the brim and continue to add fuel, the extra fuel can enter what is known as the expansion space of the tank. This space is necessary for the fuel to expand when temperatures increase, due to thermal expansion. Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.
Gasoline, having a higher coefficient of thermal expansion compared to the steel tank, will expand more as it heats up. If a tank is overfilled, there is no room left for the fuel to expand, and this can cause spills or overflow as the temperature rises, leading to environmental and safety concerns.
Moreover, the extra fuel can saturate the charcoal canister, which is part of the vehicle's evaporative emission control system. This system is designed to capture and recycle fuel vapors before they can escape into the atmosphere, and saturating the charcoal canister can impair its ability to function properly. Thus, the correct answer to the question is: Avoid topping off the fuel tank because it can saturate the charcoal canister.