A
Dashes are where you would pause.
Therefor you would say, "The storm seems to be over (slight pause) (thinking, no, wait, it isn't) no, the sky is darkening again."
Answer:
The simple subject is the tropical wave.
Explanation:
A simple subject is something that is performing an action. It can be a person, place, or even a thing. In this sentence, the thing is the tropical wave. The tropical wave is performing an action. It is entering the Gulf of Mexico, a place. The answer is not the Gulf of Mexico because the gulf is not performing any action.