Smart traveler, expert opinion about the airport

The first piece of advice is, people should always carry a good book. It helps to pass the time as you wait for your delayed flight. Don’t forget to take a sweater or a jacket on the plane. It can get very cold on a long night flight. And then there is airline food. Take a snack (cookies or fruit) with you. Sometimes the food is late, sometimes it doesn’t arrive at all, andit’s never very good.

The words “sometimes” and “never”, in bold type in the text, are adverbs of