【简答题】电子束加工是在_____条件下,利用电子枪中产生的电子经加速、聚焦,形成高能量大密度的细电子束以轰击工件被加工部位,使该部位的材料熔化和蒸发,从而进行加工,或利用电子束照射引起化学变化而进行加工
【多选题】下列对预算定额计量单位的确定,正确的是( )
【简答题】A.Ways to reduce the costs of building dams. B.Steps to improve water use management. C.Measures to deal with worldwide water shortages. D.Attempts to handle the pressure on water supply.
【单选题】The water behind the Three Gorges Dams (三峡大坝) should be ______ higher than downstream(下游)。
【判断题】The main difference between contemporary dams and Dujiangyan system is that the former block the flow of water while Dujiangyan allows water to flow naturally.
【简答题】电子枪发射的电子经聚焦形成电子束,并打在屏幕中心的()上。
【单选题】Water __________ such as dams and canals are threatening the fish population.
【判断题】The government has to open the dams to provide more water for the people.