Las Meninas by Diego Valazquez is the painting that you are referring to. It is a rather famous painting and extremely popular for academics in the western world.
This is a subjective question. None of us will have any idea of what you learnt in the course. Please provide context next time, thanks.
Answer: The contrast in art is basically putting two opposing elements together. It's one of the basic art principles used by designers and artists all over the world. For example, you can achieve contrast by combining black and white colors when painting. This is one of the most obvious examples of contrast in art.
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Answer: Rib vaults are groin vaults with extra stone ribbing.
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