Which Factor in the Modified Drake Equation Are We Best

Multiple Choice

Which factor in the modified Drake equation are we best able to estimate?

A) N_hp, the number of habitable planets in our galaxy
B) f_civ, fraction of life-bearing planets where intelligent civilizations have at some time arisen
C) f_now, fraction of civilization-bearing planets that have intelligent civilizations on them now
D) f_life, fraction of life-bearing habitable planets which are intelligent now