Which of the following best conveys the' idea that man has been careless and unconcerned in his relationship with nature
A. He has exploited the earth' s resources.
B. He has shown little understanding of nature' s ways.
C. He has abused the earth' s resources.
D. He has not appreciated nature' s beauty and benefits sufficiently.
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我国很多政府大型项目的采购所使用价格为( )
A. 开标价格
B. 拍卖价格
C. 现货价格
D. 参考价格
What does the sentence "The air helps purify the water" illustrate
A. The ecological balance.
B. The water cycle.
C. The earth' s limited resources.
D. The life cycle.
方程代表的图形是( )
A. 抛物线
B. 椭圆
C. 双曲线
D. 直线
回馈制动的调速方式( )，配上各种调节器可组成性能指标较高的调速系统，因而在工业中得到了广泛的应用。
A. 控制功率较小，效率较高
B. 有自动灭磁装置
C. 能量可回馈电网，效率高、经济
D. 只能在n＞n0时得到制动转矩
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