These three men are going through MI6 training, trying to become secret agents. They finally get through all their written and physical tests when they are pulled aside by one of the instructors who takes them to a small room with a one way mirror in it looking into another room.
They bring the first guy's wife into that room and leave her there. The instructor then loads two rounds into a pistol, hands it to the first man and says "Go kill your wife of five years." The trainee takes the weapon, goes into the next room but comes back out 1 minute later and says "I can't do it." The instructor replies, "Then you fail out - get out."

They then bring the second guy's wife into the room and leave her there. The instructor then loads two rounds into a pistol, hands it to the second man and says "Go kill your wife of ten years." The trainee takes the weapon, goes into the next room but comes back out 3 minutes later and says "I can't do it." The instructor replies, "Then you fail out - get out." 

Finally, they bring the third guy's wife into the room and leave her there. The instructor then loads two rounds into a pistol, hands it to the third man and says "Go kill your wife of fifteen years." The trainee takes the weapon, goes into the next room where there is silence for 1 minute. Suddenly, there are two gunshot sounds followed by a huge commotion in the room. The third man came out finally, sweating profusely, and says, "Good job, moron! You gave me blanks - I had to choke the bitch!"