未知题型 Egypt Felled by FamineEven ancient Egypt's mighty pyramid builders were powerless in the face of the famine that helped bring down their civilisation around 2180 BC. Now evidence gleaned from mud deposited by the River Nile suggests that a shift in climate thousands of kilometres 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 southwards out of Ethiopia would have diminished these floods.Dwindling rains in the Ethiopian highlands would have meant fewer plants to stablise the soil. When rain did fall it would have washed large amounts of soil into the Blue Nile and into Egypt, along with sediment from the White Nile4.The Blue Nile mud has a different isotope signature from that of the White Nile. So by analysing 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 predates 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 in New York.Similar events today could be even more devastating, says team member Daniel Stanley, a geoarchaeologist from the Smithsonian Institution in Washington, D.C. '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 '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 civilisation.
未知题型 有一毛石混凝土重力式挡土墙,如下图所示,墙高为5.5m,墙顶宽度为1.2m,墙底宽度为2.7m。墙后填土表面水平并与墙齐高,填土的干密度为1.90t/m3。墙背粗糙,排水良好,土对墙背的摩擦角δ=10°。已知主动土压力系数ka=0.2;挡土墙埋置深度为 0.5m,土对挡土墙基底的摩擦系数μ=0.45。挡土墙后填土的容重γ=20kN/m3,当填土表面无连续均布荷载作用,即q=0时,试问,主动土压力En,最接近于( )项数值。A.60.50kN/mB.66.55kN/mC.90.75kN/mD.99.83kN/m
单项选择题 某妇女,28岁,平素月经规律,26~28天一次,每次持续4天,其上一次月经是10月1日。今日是10月3日,那么,她的子宫内膜变化处于