Why do we only include incremental cash flows when evaluating a project using the NPV project evaluation method?

Question $1$Select one:

a$.$

Because incremental cash flows are always positive cash inflows.

b$.$

Because the NPV of the incremental cash flows resulting from a decision is a measure of the increase in the value of the firm as a result of that decision.

c$.$

Because if a cash flow is not an incremental cash flow, it is not really a cash flow $-$ it's an accounting book entry that does not affect the value of the firm.

d$.$

Because only incremental cash flows are tax$-$deductible.