What must happen for snow crystals to form naturally? (A) Fog or mist must be sprayed into the air. (B) Northern winds must heat up in the atmosphere. C) The ground must remain at a steady temperature. (D) Water vapor must encounter dust or other particles.

To determine what must happen for snow crystals to form naturally, let's analyze the options provided: (A) Fog or mist must be sprayed into the air. - This is not a natural process for snow formation. Snow crystals form from water vapor in the atmosphere, not from artificially created fog or mist. (B) Northern winds must heat up in the atmosphere. - Heating winds would not contribute to snow crystal formation. In fact, colder temperatures are necessary for snow to form. (C) The ground must remain at a steady temperature. - While temperature plays a role in snow formation, it is not specifically required for the ground to remain at a steady temperature. Snow can form in varying conditions. (D) Water vapor must encounter dust or other particles. - This is the correct answer. Snow crystals form when water vapor in the atmosphere condenses and freezes around tiny particles, such as dust or pollen. These particles serve as nuclei for the ice crystals to form. Therefore, the correct answer is **(D) Water vapor must encounter dust or other particles.**