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!

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.