If Investment Projects Are "Mutually Exclusive", What Does That Mean

If investment projects are "mutually exclusive", what does that mean?

A) Those projects are only available to companies that don't need to borrow money for the investment
B) These projects are considered to have a higher-than-normal risk, so companies would apply a higher discount to the projected cash flows
C) If the business decides to invest in one of these projects, they cannot also invest in any of the others
D) A company would only invest in this type of project with a trusted partner, in order to share the risk