In the following valid domain calculus query, what does "exists SSN$"$ refer to$?$

drc$6$:$=$

$\{$LNAME$,$ FNAME $$

$($exists SSN$)$

$($employee $($FNAME$,-,$ LNAME, SSN$,-,-,-,-,-,-)$ and

not$(dependent(SSN,-,-,-))$

any one row of employee

any one value for the variable SSN

all values for the variable SSN

any one row of dependent