Suppose $A$ is a set. Describe the conditions under which the following statement is true. $A=A-\frac{O}{?}$ Choose the correct answer below and, if necessary, fill in the answer box to complete your choice. A$.$ This statement is only true if Asube$\frac{O}{?}$ B$.$ This statement is only true if Asub$\frac{O}{?}.$ C$.$ This statement is true for any set $A.$