We________on this project for four hours.Let's have a rest.

A．are working

B．have been working

C．worked

D．had worked

下列对CSS字母间隔表述不正确的一项是

A．语法: letter-spacing: 值

B．允许值: normal | 长度

C．初始值:none

D．字母间隔属性定义一个附加在字符之间的间隔数量