Answer
The set 1 has each $x$ -coordinate paired with only one $y$ -coordinate.
Work Step by Step
Consider the two sets of ordered pairs,
Set 1: $\left\{ \left( 1,5 \right),\left( 2,5 \right) \right\}$
Set 2: $\left\{ \left( 5,1 \right),\left( 5,2 \right) \right\}$
In set 1, both $x$ -coordinates are different (1 and 2), and $y$ -coordinates are the same (5).
So, in set 1 only one $y$ -coordinate is 5.
Thus, set 1 has each $x$ -coordinate paired with only one $y$ -coordinate.
Check for set 2:
Set 2: $\left\{ \left( 5,1 \right),\left( 5,2 \right) \right\}$
In set 2, both $x$ -coordinates are the same (5) and the $y$ -coordinates are different (1 and 2).
So, in set 2, the $y$ -coordinates are not the same.
Hence, set 1 has each $x$ -coordinate paired with only one $y$ -coordinate.
