编写一个打字母练习软件，随机从屏幕上方掉落字母，键盘点击该字母后消失，否则落到屏幕最下方，扣分，敲击成功加分，利用Timer控件实现字母的随机出现。-Write a letter playing practice software, random letters falling　the top of the screen, click on the letter keyboard disappears, otherwise it falls bottom of the screen, points, hit points for success, use Timer controls to achieve random letters.