问题
选择题
Great changes ______ in China in the past 20 years.
A．took place
B．have taken place
C．were taken place
D．have been taken place
答案
答案：B
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Great changes ______ in China in the past 20 years.
A．took place
B．have taken place
C．were taken place
D．have been taken place
答案：B
·
，太阳质量Ms=2.0×1030kg，地球质量ME=5.98×1024kg，太阳和地球间的距离r=1.5×108km，则地球对太阳的引力为多大？（计算结果保留两位有效数字）