Mathematics

Problem

Let

A=\{1,2,3,4\}

and

B=\{1,2,3,4,5\}

. The relation

R=\{(1,2),(1,3),(2,4),(3,4),(3,5),(4,5)\}

is a subset of

A\times B

. What is the greatest possible number of ordered pairs that can be retained in

R

so that the remaining relation becomes a function from

A

B

? a)

3

6

4

5