完成四则运算功能，先通过打开文件，打开保存的题目文件（程序附带有一个data.txt），点OK按钮就可开始答题，点next回答下一道题，点check核对答案。-Four arithmetic functions, first by opening the file, open the saved topic file (the program comes with a data.txt), and click OK button can begin to answer the point nex