问题
选择题
The invention of steam engine greatly sped up the industrial revolution, which _____ a lot to the take-off of the world. [ ]
A. led
B. contributed
C. constructed
D. exposed
答案
答案：B
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0.5T/s在变化，光滑的水平导轨宽为L=0.8m，电阻不计，在导轨上d=1m处有一导体棒ab，其电阻r=0.2Ω，并用水平细线通过定滑轮吊着质量为M=2kg的重物，固定电阻R=0.8Ω，求经过多长时间重物将被提起。（取g=10m/s2）