18.【单选题】（）是人类生存和发展的最基本的活动，是人类社会活动的本质，是人的认识产生和发展的基础，也是真理与价值统一的基础。

It took him a full day to ____ those bags.

The captain needs to be ____ of this.

I am terribly sorry I can＇t ____ your invitation due to my business.

The e-ticket is very ____ for everyone.

You can check the ____ of Spring Airlines for the flight to Guangzhou.

There are verious ____ of movies on the plane.

Can you tell the ____ between the boarding pass and the ticket?

Which one should be taken out of packed baggage to scan at the security check point?

Why is it advised to take excess clothes off when waiting for security check?

Which is the item that won＇t set off metal detectors?