Why might discounted cash flow valuation be difficult to do for the following types of firms?

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 cyclical firm during a recession.

d. A troubled firm that has made significant losses and is not expected to get out of trouble for a few years.

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

f. A firm that owns a lot of valuable land that is currently unutilized.