The air heating the sidewalk The sidewalk reflecting sunlight into the air The wind causing the ice on the sidewalk to melt The sunlight making the sidewalk warmer than the air
I gave my students this multiple choice question: A thermometer shows that the outside air temperature is colder than the temperature at which water turns to ice. However, ice on the sidewalk melts. What probably caused this?

Only one answer is correct among these 4 choices:
- The air heating the sidewalk
- The sidewalk reflecting sunlight into the air
- The wind causing the ice on the sidewalk to melt
- The sunlight making the sidewalk warmer than the air

Could you tell me which one is correct?

The sunlight making the sidewalk warmer than the air