For Which of These Factors in the Drake Equation Do

Multiple Choice

For which of these factors in the Drake equation do astronomers 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