It is adverb... it is mostly called adverbial clause
Answer:
It has decreased because over time because water creates caverns and also polution can kill alge in the water witch can cause the water flow to decrease in folucules.
Explanation:
Mysterious Decline of the Maya. From the late eighth through the end of the ninth century, something unknown happened to shake the Maya civilization to its foundations. One by one, the Classic cities in the southern lowlands were abandoned, and by A.D. 900, Maya civilization in that region had collapsed.
Hope this helps. :P
Its A , because it separates the sentence the best.