Can we define the following sets by extension? A = {the members of the River Plate club in Argentina}, B = {the stars of the universe}, C = {positive fractions with denominator 3}, D = {even numbers}.

a) We can define set A by extension if we have access to the club's membership records. We cannot define set B by extension because it is impossible to enumerate all the stars in the universe. Sets C and D cannot be defined by extension because they are infinite sets.
b) Sets C and D can be defined by comprehension. Set C can be defined as {x | x is a positive fraction and the denominator of x is 3}. Set D can be defined as {x | x is an even number}.