Cell lines A and B both survive in tissue culture containing serum but do not proliferate. Addition of Factor F stimulates proliferation of cell line A but not cell line B. Cell line A produces a normal receptor protein (R), and, in order to see what the role of R might be, you introduce this receptor protein into cell line B, using recombinant DNA techniques. You then test all of your various cell lines in the presence of serum for their response to factor F, with the results summarized in Table below.
Table

Cell line Response
A - Factor F Cells do not proliferate
A + Factor F Cells proliferate
B - Factor F Cells do not proliferate
B + Factor F Cells do not proliferate
B + receptor R - Factor F Cells proliferate
B + receptor R + Factor F Cells proliferate

Which of the following can be concluded from your results? Please describe your choices.

a. Binding of Factor F to its receptor is required for proliferation of cell line A but not cell line B.

b. Binding of receptor R to its ligand is required for proliferation of cell line B but not cell line A.

c. Receptor R probably does not bind to Factor F.

d. Cell line B does not normally have a receptor for Factor F.