Antarctica has a thick ice cover and the highest average elevation of all the continents.

does the combined new sentence use a compound subject, compound verb, or compound object?

The subject here is simple" it's just "Antarctica" .

The verb is also not compound, it's a simple verb: "has".

The compound part of the sentence is the object: it is: "a thick ice cover and the highest average elevation of all the continents"

so the correct answer is "compound subject"

It is a good way to protect your artwork.

It was initially part of the frame and mat of wood.

During the renaissance, it was made out of paper.

At the beginning of the 19th century, cardboard was invented. Later on, mats for artwork were mass produced out of cardboard.

Nowadays, there is a digital mat that can be used by professional designers.

Explanation:

Hope this helps!