[单项选择]

At some time around 2300BC, give or take a century or two, a large number of the major civilizations of the world collapsed, in Mesopotamia, Egypt, Israel, Anatolia, and Greece, as well as in Afghanistan and China. All of them the first urban civilizations fell into rain at more or less the same time. A thousand years later, around 1200BC, many of the civilizations of the same regions again collapsed at about the same time.
The reasons for these widespread and apparently simultaneous disasters which coincided with changes to cultures and societies elsewhere, such as in Britain, have long been a fascinating mystery. Traditional explanations included warfare, famine, and more recently systems collapse, but the apparent absence of direct archaeological or written evidence for causes, as opposed to effects, has led many archaeologists and historians into a resigned assumption that no definite explanation can be found.
Over the past 15 years, however, a new type of &rs
A. comets.
B. comets or material from comets.
C. earthquakes, tidal waves, and volcanic eruptions.
D. climate change.

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