For what reasons might discounted cash flow valuation be difficult for the following types of firms in the situations stated?

A.A private firm, where the owner is planning to sell the firm.

B.A biotechnology firm with no current products or sales, but with several promising product patents in the pipeline.

C.A firm in the process of restructuring, where it is selling some of its assets and changing its financial mix.

D. A firm which owns a lot of valuable real estate that is currently not utilized.