单项选择题

土壤中微生物的活动不会对重金属的生物有效性产生影响。 A. 对 B. 错

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单项选择题

Byimproving the electric light bulb, Thomas Edison made it possible for homes or individual houses to be electrified at a time when almost everybody was _________, since electric light was also a much safer form of illumination than gas or candles or oil lamps.

A．using kerosene
B．using coal
C．using gas
D．using charcoal

单项选择题

题4-2-3、电流源可以起一个电阻的作用，而且不消耗（）的电压余度。

A．过高
B．恒定
C．变化
D．过低