If y varies directly as x, and y is 12 when x is 1.2, what is the constant of variation for this relation?
1
10
10.8
14.4

sample answer: you can determine whether each element of the domain is paired with exactly one element of the range. for example, if given a graph, you could use the vertical line test; if a vertical line intersects the graph more than once, then the relation that the graph represents is not a function.