阅读以下材料完成36-40题Egypt Felled by FamineEven ancient

阅读以下材料完成36-40题

Egypt Felled by Famine

Even ancient Egypt’s mighty pyramid builders were powerless in the face of the famine that helped bring down their civilization around 2180 BC.Now evidence gleaned（搜集）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 their crops.But any change in climate that pushed the African monsoons（季风）southwards out of Ethiopia would have diminished these floods.

Dwindling（使减少）rains in the Ethiopian highlands would have meant fewer plants to establish the soil.When rain did fall it would have washed large amounts of soil into the Blue Nile and into Egypt,along with sediment（沉积物）from the White Nile.

The 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 the Blue Nile mud in the river would be relatively high.He found that one of these periods,from 4,500 to 4,200 years ago,immediately predated the fall of the Egypt’s old Kingdom.

The weakened waters would have been catastrophic for the Egyptians."Changes that affect food supply don’t have to be very large to have a ripple effect in societies,"says Bill Ryan of the Lamont Doherty Earth Observatory（天文台）in New York.

"Similar events today coald be even more devastating,"says team member Daniel Stanley,a geoarchaeologist from the Smithsonian Institution in Washington, D.0."Anything humans do to shift the climate belts woulld have an even worse effect along the Nile system because the populations have increased dramatically."

Why does the author mention"pyramid builders"?

A.Because they once worked miracles.

B.Because they were well-built.

C.Because they were actually very weak.

D.Because even they were unable to rescue their civilization.