在下列房地产市场指标中，属于市场交易指标的有（）。

A．空置量

B．房地产价格指数

C．平均建设周期

D．吸纳率

E．失业率

WAP uses (71) , which includes the Handheld Device Markup Language (HDML) developed by Phone.com. WML can also trace its roots to eXtensible Markup Language (XML). A markup language is a way of adding information to your (72) that tells the device receiving the content what to do with it. The best known markup language is Hypertext Markup Language. (73) HTML, WML is considered a meta language. Basically, this means that in addition to providing predefined (74) , WML lets you design your own markup language (75) . WAP also allows the use of standard Internet protocols such as UDP, IP and XML.

（74）处填（）。

A．tags

B．certificates

C．messages

D．authentication codes