Why must aggregate bulk materials (such as sand, gravel, soil, or crushed rock) transported in open dump trucks in Bahrain be covered with a secure tarpaulin?

1. To keep the stones warm in winter
2. To prevent loose stones and dust from blowing off, which breaks trailing car windscreens, blinds motorists, and creates road hazards
3. To hide the cargo from road tax inspectors
4. Tarpaulins are strictly prohibited on open dump trucks