In a function, every value of x must have a unique value of y. But every y value is NOT required to have a unique x value.
Only #2 is a function.
2) { (6,7), (8,9), (2,7), (5,10) } ---> y values are repeated but not x values
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1) { (3,6), (3,5), (6,5), (7,5) }
---> When x== 3, there are two y values
3) { (3,6), (5,9), (4,10), (2,8), (3,9) }
---> When x== 3, there are two y values
4) { (1,5), (1,8), (1,13), (1,6) }
---> When x== 1, there are 4 y values
