The Number of Technological Civilizations in Our Galaxy Is Limited

Multiple Choice

The number of technological civilizations in our galaxy is limited by

A) the number of stars in our galaxy.
B) the fraction of stars with planets.
C) the number of planets per star in habitable zones longer than a few billion years.
D) the fraction of suitable planets where technical life forms evolve.
E) the number of stars in our galaxy, the fraction of stars with planets, the number of planets per star in habitable zones longer than a few billion years, and the fraction of suitable planets where technical life forms evolve.