Final answer:
The correct answer is (a) North, as winds typically shift to come from the north or northwest behind a cold front in a wave cyclone in the Northern Hemisphere.
Step-by-step explanation:
The wind direction at St. Louis, on the Missouri-Illinois border, was generally from the north as would be expected behind the cold front in a wave cyclone. Winds that were previously coming from the south (bringing warmer air) will turn and come from the north or northwest (bringing colder air), consistent with the counterclockwise flow around a low-pressure system and the influence of the Coriolis force.
The correct answer to this question is (a) North. This is because, in the Northern Hemisphere, the characteristic rotation of a wave cyclone is counterclockwise. When a cold front passes, the wind direction typically shifts. Winds that were previously coming from the south (bringing warmer air) will turn and come from the north or northwest (bringing colder air), consistent with the counterclockwise flow around a low-pressure system and the influence of the Coriolis force.