地心坐标系是以什么为中心的坐标系统？

Area A in the following figure represents A、producer su...

单项选择题Area A in the following figure represents
A、producer surplus to new producers entering the market as the result of an increase in price from P1 to P2.
B、the increase in consumer surplus that results from an upward-sloping supply curve.
C、the increase in total surplus when sellers are willing and able to increase supply from Q1 to Q2.
D、the increase in producer surplus to those producers already in the market when the price increases from P1 to P2.

参考椭球体是用来

未知题型参考椭球体是用来

若某网络中的所有支路电压和支路电流都有唯一解，且已知某支路k的电流ik或电压uk ，则可以用一个电压等于uk的...

单项选择题若某网络中的所有支路电压和支路电流都有唯一解，且已知某支路k的电流ik或电压uk ，则可以用一个电压等于uk的（  ）或电流为ik的（  ）去等效替代这条支路，替代后网络其他部分的电压和电流值保持不变。 A.
电压源；电流源
B.
电流源；电压源 
C.
电流源；电流源
D.
其它选项均不正确