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, poin