My Answer:
Water OR letter D.
My Explanation:
It says in the given "In the dry climate of the Middle East, __________ is valuable because there is a limited supply."
That is because there was/is barely any rainfall there and so it is always SO humid, that is how and why WATER or RAIN is so valuable.
This is the correct answer on edge 2020/2021 it might not be the same for everyone sorry if this isn't the correct answer for you. :( :)