In the following valid tuple calculus query, what does "exists $D"$ refer to$?$

$trc1$:$=$

$\{$E$.$fname, E$.$lname$,$ E$.$address I employee$($E$)$ and

$($exists D$)$

$($department $($D$)$ and

D$.$dname$=$'Research' and D$.$dnumber$=$E$.$dno$)\}$;

any one tuple

all tuples

no tuple

a pair of tuples