As a presenter, you are able to set Activity to public or private.
On the Settings screen, under Activity in the Preferences section, you can elect to "Keep posts private". When you do this, only you as the presenter will see comments and questions from participants.
If there is a particular post you would then like to make public, you can do so by selecting "Make public" next to that post. Once a post has been made public, it can not subsequently be made private again.
If you deselect "Keep posts private", all participants in your zeeting will see new posts. That said, as the presenter, you are always able to delete posts.