Slang idioms
Q1
He finally found the ..... link. The Germans had been able to shoot down most of the English's planes when they went to bomb the Germans. Some in the English bomber command thought that there was an intelligence leak who was responsible but that was not the case. It turned out that the Germans set up a radar station that could detect the incoming planes.
(*) forging
(*) missing
(*) elusive
(*) foggy