Question

what is the main idea presented in this excerpt?
as fewer people get the flu virus, the flu shot becomes less and less useful.
as fewer people get the flu virus, the flu shot becomes more and more useful.
as fewer people get the flu shot, it becomes more effective, but as more people get it, the flu shot becomes less effective.
as fewer people get the flu shot, it becomes less effective, but as more people get it, the flu shot becomes more effective.

Explanation:

The flu - shot's effectiveness is related to the number of people getting it. Generally, the more people get the flu - shot, the more effective it is in preventing the spread of the flu virus. When fewer people get the flu - shot, its overall effectiveness in protecting the population is reduced.

Answer:

As fewer people get the flu shot, it becomes less effective, but as more people get it, the flu shot becomes more effective.