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Cargo covers serve multiple purposes, making the choice of both options appropriate. Primarily, they are designed to protect the cargo itself from exposure to the elements, which can include rain, snow, dust, and UV rays. This protection not only helps maintain the integrity and quality of the cargo but also ensures that it is safely secured and less likely to spill during transport.

Moreover, cargo covers can indeed prevent the spillage of cargo, which subsequently protects people nearby and prevents environmental hazards. Additionally, the presence of cargo covers can enhance visibility by providing a clear delineation between the cargo and the surrounding environment, making the load more noticeable to other drivers and ensuring that it is correctly identified on the road.

Lastly, while protecting against theft is a vital consideration, cargo covers contribute indirectly to theft prevention by making it more difficult for would-be thieves to access the contents quickly, thus acting as a deterrent.

In summary, cargo covers fulfill several critical roles in transport safety, protection, and visibility, which is why the answer reflects that both stated purposes are correct.