题目内容：
Ancient Egypt Brought Down by Famine Even ancient Egypt's mighty pyramid （金字塔） builders were powerless in the face of the famine （饥荒） that helped bring down their civilization around 21-80 BC. Now evidence collected from mud deposited by the River Nile suggests that a shift in climate thousands of kilometers to the south was ultimately to blame--and the same or worse could happen today.The ancient Egyptians depended on the Nile's annual floods to irrigate !heir crops.But any change in climate that pushed the African monsoons （季风） southwards out of Ethiopia would have reduced these floods.
Declining rains in the Ethiopian highlands would have meant fewer plants to stabilize the soil. When rain did fall it would have washed large amounts of soil into the Blue Nile and into Egypt, along with sediment （沉积） from the White Nile.Blue Nile mud has a different isotope （同位素） signature from that of the White Nile.So by analyzing isotope differences in mud deposited in the Nile Delta, Michael Krom of Leeds University worked out what proportion of sediment came from each branch of the river.
Krom reasons that during periods of drought, the amount of Blue Nile mud in the river would be relatively high. He found that one of these periods, from 4500 to 4200 years ago, immediately came before the fall of the Egypt's Old Kingdom.
The weakened waters would have been disaster for the Egyptians. "Changes that affect food supply don't have to be very large to have a ripple （波浪） effect in societies," says Bill Ryan of the Lamont Doherty Earth Observatory in New York.
Similar events today could be even more devastating, says team member Daniel Stanley, a scientist from the Smithsonian Institution in Washington DC. "Anything humans do to shift the climate belts would have an even worse effect along the Nile system today because the populations have increased dramatically."Why does the author mention "Egypt's mighty pyramid builders". A．Because they once worked miracle
B．Because they were well-buil
C．Because they were actually very wea
D．Because even they were unable to rescue their civilizatio
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