The _____ is a geologically brief period about 540 million years ago during which fossil evidence indicates Earth life became complex and diverse.
A) Cambrian explosion
B) Paleozoic era
C) Renaissance
D) Ordovician
Question 2
Which of the following does N represent in the Drake equation, Nc = N fp nHZ fL fI fs?
A) The average number of planets in a solar system
B) The fraction of stars that have planets
C) The number of stars in our galaxy
D) The fraction of suitable planets on which life begins