Need help ...

Q: An organization has four projects to choose from: Project A with a NPV $45,000, Project B with a NPV $60,000, Project C with a NPV $75,000, Project D with a NPV $30,000. What is the oppurtunity cost of selecting Project C?

A. $60,000

B. $135,000

C. $30,000

D. $45,000

Please help in finding the right answer, and also how should we solve if more than 2 projects scenario is given and asks us to find out the oppurtunity cost.