20.【单选题】毛泽东所指出的：“认识有待于深化，认识的感性阶段有待于发展到理性阶段——这就是认识论的辩证法。”这句话表明：（）。

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?

What do you need to remove at the security check point according to the text?

Which one is NOT the way to find your airline according to the text?