Which Factor in the Modified Drake Equation May Well Turn

Multiple Choice

Which factor in the modified Drake equation may well turn out to be the primary factor in determining the current number of intelligent civilizations in our galaxy?

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