When Is Subtraction Associative?

Algebra Level 1

$\large (a-b) - c = a - (b - c)$

Suppose $a, b$ and $c$ are numeric variables. Under what conditions is the equation above true?

Always true Only true when at least one of the variables is equal to 0 Only true when

c=0

Only true when all of the variables are equal to 0 Never true