If an air mass is sinking, it must be A. cold and less dense
B. warm and more dense
C. cold and more dense
D. warm and less dense

The awnser to your question is C.

Explanation:

The colder matter is the atoms get closer therefore making the matter more dense. A. is half correct but if the matter is cold it wouldn't be more dense because when something is less dense it goes above whatever is around it. Take for example water and oil. They seperate into different layers due to density.     Hope this helps. :)

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