一个“黑板”的程序，界面上有粉笔和板擦，用户可以使用粉笔来划线，或者使用板擦来擦除黑板上书写的内容。同时黑板还支持文字输入。-A " blackboard" process, interface with chalk and eraser, the user can use chalk to dash, or use eraser to erase the contents of the writing on the blackboard. At the same time B