The following is an incomplete SQL query that finds the sids of suppliers who supply red parts but not green parts:

SELECT DISTINCT C$.$sid

FROM Catalog $C,$ Parts $P$

WHERE C$.$pid $=$ P$.$pid AND P$.$color $=$ 'Red'

$?$

SELECT DISTINCT C$1.$sid

FROM Catalog C$1,$ Parts P$1$

WHERE C$1.$pid $=$ P$1.$pid AND P$1.$color $=$ 'Green'

The missing operator $($at $?)$ is:

a$.$ INTERSECT

b$.$ UNION

c$.$ EXCEPT

d$.$ JOIN

e$.$ BUT