question: How do gases prevent evaporation of the filament ?
answer: The presence of gas molecules knocks the liberated tungsten atoms back to the filament
turker_answer: The presence of gas molecules knocking the liberated tungsten atoms back to the filament is how gases prevent evaporation of the filament .
rule-based: Gases prevent evaporation of the filament by the presence of gas molecules knocks the liberated tungsten atoms back to the filament .