问题如下：
2. The optimal subset of the six projects that Schoeman is considering consists of Projects:
选项：
A.1 and 5.
B.2, 3, and 4.
C.2, 4, 5, and 6.
解释：
A is correct.
To help the selection process, use the profitability index for each project, which shows the total present value per dollar invested.
Try to incorporate the high PI projects into the budget using trial and error. These trials include the following:
Among the sets of projects suggested, the optimal set is the one with the highest NPV, provided its total outlay does not exceed C$40 million. The set consisting of Projects 1 and 5 produces the highest NPV.
不是很明白算答案里面算pi是干什么用的呢？
