为了省去擦黑板的麻烦,解决擦黑板造成的污染,我设计了这样一种新型黑板:它是黑色特制帆布制成的,用上下两个滚轴固定在墙壁上。当黑板的一面写满粉笔字时,老师只要一按按钮,这面的黑板就滚到后面去了,黑板靠墙的那一面已被一排刷子刷掉粉笔字。黑板下面有 In order to eliminate the hassle of wiping the blackboard and solving the pollution caused by the blackboard, I designed a new type of blackboard: it is made of black special canvas and fixed on the wall with two upper and lower rollers. When one side of the blackboard was written with chalk, the teacher rolled the back of the blackboard as soon as he clicked the button, and the blackboard was stripped of chalk by a row of brushes on the side of the wall. Below the blackboard there