A warm and dry air mass would probably have begun
A.
over a tropical land region.
B.
over a polar land region.
C.
over a tropical ocean region.
D.
over a polar ocean region.

A. over a tropical land region.

Explanation:

Warm, so it's tropical and not polar. Dry, so it's gotta be one of the air masses over a land region and not an ocean region. A warm and dry air mass would probably have begun... over a tropical land region!

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i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.