Answer : A
Find the names of all suppliers who have supplied non-blue part.
The sub-query “SELECT P.pid FROM Parts P WHERE P.color<> ‘blue’” gives pids of parts which are not blue.
The bigger subquery “SELECT C.sid FROM Catalog C WHERE C.pid NOT IN (SELECT P.pid FROM Parts P WHERE P.color<> ‘blue’)” gives sids of all those suppliers who have supplied blue parts.
The complete query gives the names of all suppliers who have supplied a non-blue part