The temperature change is caused by the advection of warmer or colder air into the area.
While it is true that warm or cold advection does cause changes in temperature (ever experience a cold front?), this hypothesis would predict that temperature changes should be randomly distributed throughout the day rather than systematically rise during the morning and fall during the evening.
This hypothesis does explain many temperature changes, and is easily tested by looking at a weather map, but it does not explain the regular diurnal temperature cycle.
If you picked this hypothesis, go back to the list of hypotheses and select another one.