31.【单选题】发展的实质是新事物的产生和旧事物的灭亡。新旧事物的区别在于（）。

A ____ of 20,000 people came to the concert.

What＇s the ____ rate between US dollar and Pound?

The customs officer caught a ____ dealer at the airport.

____ is an item that is not legally brought into the country.

She is very ____ and always cares about other people＇s feeling.

The bags are stored in the cargo ____ of the aircraft.

Some airlines provide baby ____ on their international routes.

How many vacant seats are ____ on this flight?

This door should only be used in an ____.

Please fasten your ____ while the captain prepares for takeoff.