For Which Factor in the Drake Equation Do We Actually

Multiple Choice

For which factor 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) fl, the fraction of Earthlike planets on which life actually arises
C) fᵢ, the fraction of life-forms that evolve into intelligent species
D) fc, the fraction of intelligent species that develop adequate technology and then choose to send messages out into space