Which Sets of Ordered Pairs Are Functions $A = \{ ( 4,3 ) , ( 17,29 ) , ( 18,52 ) , ( 4,19 ) \}$

Which sets of ordered pairs are functions? $A = \{ ( 4,3 ) , ( 17,29 ) , ( 18,52 ) , ( 4,19 ) \}$ $B = \{ ( 19,4 ) , ( 52,18 ) , ( 29,17 ) , ( 3,4 ) \}$ $C = \{ 69,82,44,87 \}$

A) $A$ is not a function; $B$ is a function; $C$ is a function.
B) $A$ is not a function; $B$ is a function; $C$ is not a function.
C) $A$ is a function; $B$ is not a function; $C$ is not a function.
D) $A$ is a function; $B$ is a function; $C$ is not a function.
E) $A$ is not a function; $B$ is not a function; $C$ is not a function.