Find out which of the following sets are groups under addition (please explain why):
a) The set of rational numbers (including 0 = 0/1) in lowest terms whose denominators are odd
b) The set of rational numbers in lowest terms whose denominators are even together with 0.
c) The set of rational numbers of absolute value >= 1 together with 0
d) The set of rational numbers with denominators equal to 1 or 2