What is the primary danger when riding a motorcycle behind a large truck on an unpaved gravel mountain road?

1. Heavy dust clouds reduce visibility to zero, and the truck wheels throw gravel stones that can shatter your visor or injure your body
2. Following close behind keeps you out of the dust, because the plume rises above you
3. The truck's wheels compact the surface, making the line directly behind it the safest
4. The only risk is that the machine becomes dirty and needs washing