To analyze the student's weather observations in the context of air mass interactions, let's examine the observations:
- Day 1: Sunny and warm - This is typical of a warm air mass.
- Day 2: Cloudy and rainy - This suggests that moisture is being introduced, likely due to a warm front or some other moisture-rich air mass.
- Day 3: Drop in temperature - This indicates a transition, likely associated with the arrival of a cooler air mass.
- Day 4: Clear skies and cold temperatures - This suggests the presence of a cold air mass after the previous day's drop in temperature.
Given these observations, the best explanation is that a cold front moved into the area. A cold front typically brings a drop in temperature and can lead to rainy or stormy weather as it displaces a warmer air mass.
Therefore, the correct response is: A cold front moved into the area.