问题如下图：
选项：
A. 
B. 
C. 
解释：
想问老师，这里有两个P，问题又问到交叉弹性，应该怎么判断用哪个价格呢？谢谢。
NO.PZ2016010802000026 问题如下 The market manfunction for four-yeprivate universities is given the equationQr =84−3.1Ppr+0.8I+0.9PpuwhereQr is the number of applicants to private universities per yein thousan, Ppris the average priof private universities (in thousan of US, I is the householmonthly income (in thousan of US, anPpu is the average priof public (government-supporte universities (in thousan of US. Assume thPpr is equto 38, I is equto 100, anPpu is equto 18.The cross-prielasticity of manfor private universities with respeto the priof public universities is closest to: A.0.3. B.3.1. C.3.9. is correct.From the manfunction:ΔQr/ΔPpu=0.9(the coefficient in front of Ppu )Solve for Qr = 8r−3.1Ppr+0.8I+0.9Ppu =84−3.1(38)+0.8(100)+0.9(18)=62.4Ppu = 18, prielasticity of man(ΔQr/ΔPpu)(Ppu/Qr)=(0.9)(18/62.4)=0.3考点交叉价格弹性解析题目给出了需求函数的表达式，带入数字，先求的私立大学的需求量，即Qr = 8r−3.1Ppr+0.8I+0.9Ppu =84−3.1(38)+0.8(100)+0.9(18)=62.4之后我们再利用交叉价格弹性公式求解，即，(ΔQr/ΔPpu)(Ppu/Qr)=(0.9)(18/62.4)=0.3 算出来是0.259，直接就约等于0.3？？是不是有点太多了
0.9*0.2885 只要选最接近的就行了吗
老师，想问在这类题目中，斜率应该怎么判断呢？因为考交叉提到两个斜率，不知道怎么判断，谢谢。
怎样在现实的环境中理解计算出的交叉弹性？请举例子。
