Pick all that apply

• 1Choice 1.

  post is synchronous and returns only after every registered observer has finished

• 2Choice 2.

  Observer callbacks run on the thread that called post, so if you post from the background you have to get back to main before updating UI

• 3Choice 3.

  With the closure-based addObserver(forName:object:queue:using:), unless you keep the returned token and remove it yourself, the observer stays alive

• 4Choice 4.

  NotificationCenter.default can exchange notifications with other processes (app extensions)

• 5Choice 5.

  Passing .main as the queue parameter runs the callback on the main queue