The system on which you breath.
Tropism is a descriptive reaction of a certain stimuli on how an organism responds to the confronted stimuli which can be called positive tropism or negative tropism.
<span>In positive tropism as the proposed answer </span>toward <span> involves the organism leaning forward or direct to the acting stimuli whereas the negative tropism is exactly the opposite. </span>
I'm almost sure your answer is going to be "Because the sun's mass creates a gravitational force that pulls earth towards it."
correct me if I am wrong.
Sorry if this is wrong but a closed circulatory system is blood that is contained inside blood vessels, circulating unidirectionally (in one direction) from the heart around the systemic circulatory route, then returning to the heart again.