For Which of the Following Factors in the Drake Equation

Multiple Choice

For which of the following factors in the Drake equation do we actually have observational knowledge,rather than just a speculative estimate?

A) R_*,the rate at which solar-type stars form in the Galaxy
B) f_l,the fraction of Earthlike planets on which life actually arises
C) f_i,the fraction of life forms that evolve into intelligent species
D) f_c,the fraction of intelligent species that develop adequate technology and then choose to send messages out into space