He picked up the bunch of keys from the floor, and twirling them round in his fingers finally selected one, very bright and shining, which he tried in the lock of the purple despatch-case. 
He picked the dull key. 
mnli hypothesis: He picked the dull key.  premise: He picked up the bunch of keys from the floor, and twirling them round in his fingers finally selected one, very bright and shining, which he tried in the lock of the purple despatch-case. 