问题如下：
The following equations are given for a hypothetical economy:
C = 2,000 + 0.7(Y – T) Consumption function
I = 400 + 0.2Y – 30r Investment function
G = 1,500 Government spending
(X – M) = 1,000 – 0.1Y Net export function
T = –200 + 0.3Y Tax function
Based on these equations, determine the combinations of aggregate income (Y) and the real interest rate (r) that are consistent with equating income and expenditure. That is, find the equation that describes the IS curve.
选项：
A.Y = 12,292.7 – 73.2r.
B.Y = 15,592.7 – 73.2r.
C.Y = 12,292.7 – 53.8r
解释：
A is correct.
Starting with the basic GDP identity Y = C + I + G + (X – M) and substituting for each expenditure component using the equations above gives
Y = 2,000 + 0.7(Y – T) + 400 + 0.2Y – 30r + 1,500 + 1,000 – 0.1Y
Substituting in the tax equation and solving for Y, we get
Y = 2,000 + 0.7(Y + 200 – 0.3Y) + 400 + 0.2Y – 30r + 1,500 + 1,000 – 0.1Y
= 5,040 + 0.59Y – 30r
Y = 12,292.7 – 73.2r
The final equation is the IS curve. It summarizes combinations of income and the real interest rate at which income and expenditure are equal. Equivalently, it reflects equilibrium in the goods market.
考点：IS curve
解析：从基本的GDP等式Y = C + I + G + (X M)开始，用上面给出的方程替换每个支出部分，即：
Y = 2,000 + 0.7(Y – T) + 400 + 0.2Y – 30r + 1,500 + 1,000 – 0.1Y
再代入税收方程求解Y，得到
Y = 2,000 + 0.7(Y + 200 – 0.3Y) + 400 + 0.2Y – 30r + 1,500 + 1,000 – 0.1Y
= 5,040 + 0.59Y – 30r
Y = 12,292.7 – 73.2r
最后一个方程是is曲线。它总结了收入与支出相等时的实际利率的组合。同样，它反映了商品市场的均衡。
因此只有A正确。
请问为什么不是G-T？
