The fifth week was the toughest for Katie Couric, whose viewership on the "CBS Evening News" has dropped each week since her debut the day after Labor Day. Her broadcast averaged 7.04 million viewers last week, third to NBC's "Nightly News" (8.56 million) and ABC's "World News" (7.97 million), according to Nielsen Media Research. CBS points out that the "CBS Evening News" is the only one of the three network newscasts with more viewers last week than the same week a year earlier. NBC's margin of victory last year was 2.2 million. "Where we were last week or even in...