When a road tanker carrying liquefied compressed gas (such as LPG) is exposed to external fire impingement, what is the primary purpose of applying large volumes of cooling water spray to the upper shell of the tank?

1. To cool the steel shell in the vapor space to prevent metal softening, overpressurization, and catastrophic BLEVE (Boiling Liquid Expanding Vapor Explosion / 沸腾液体扩展蒸气爆炸)
2. To wash the road dust off the vehicle logo for clear identification
3. To fill the interior of the tank with water until it overflows onto the highway
4. To generate massive clouds of steam to hide the truck from traffic cameras