Let r be a relation defined on the set of natural numbers n as follows: r=(x,y)∈N²∣2xy. The relation r can be expressed as:

a) r=(1,2),(2,4),(3,6),…
b) r=(1,1),(2,2),(3,3),…
c) r=(1,3),(2,6),(3,9),…
d) r=(1,4),(2,8),(3,12),…