To determine what might have happened as Trevor observed the air pressure change, it's important to understand how barometric pressure relates to weather systems. A falling barometer generally indicates that a low pressure system is moving in, which often leads to stormy weather. Conversely, a rising barometer indicates that a high pressure system is moving in, which is typically associated with clearer skies.
Based on this understanding:
- If Trevor observed a decrease in air pressure, it likely indicates the arrival of a low pressure system (choice D).
- If he observed an increase in air pressure, it suggests a high pressure system was moving into the area (choice A).
Since the question doesn't specify whether the barometric pressure was rising or falling, we can only choose from the given options. If we take it into account that noticing a change usually means a significant shift per standard interpretation of weather changes, it's common to interpret this as a low pressure system encroaching.
So, the most plausible answer, if we assume Trevor noticed a decrease in pressure, would be:
D. A low pressure system moved into the area.