I understand why answer choice (C) could be a correct answer to this necessary assumption problem; if you negate choice (C) to say the frequency of invalid warnings will cause pilots to IGNORE warnings, then the system itself becomes "useless" in maintaining the passengers' safety, thereby destroying the argument that the system that alerts pilots of possible collisions makes planes with this system to be safer than those planes that don't have the system..
However, I'm having a hard time eliminating answer choice (E) as a wrong answer - if you negate it, it states that the greatest risk for passengers is NOT that of a midair collision. If that's true, then wouldn't it destroy the argument that having the system makes passengers on those planes with the system safer? Because if collisions aren't the greatest risk, then having the system wouldn't necessarily make having it safer either....
If anyone could please help, I would be really grateful!
Thanks so much!