Consider the functions  f1, f2, f3 and f4 given by the following diagrams In Fig 1 2, we observe that the images of distinct elements of X1 under the function
f1 are distinct, but the image of two distinct elements 1 and 2 of X1 under f2 is same,
namely b Further, there are some elements like e and f in X2 which are not images of
any element of X1 under f1, while all elements of X3 are images of some elements of X1
under f3