A friend of yours is studying at home and has asked you to recommend a learning tool. 你的一位朋友最近在家自学，想请你推荐一个学习工具。
Write a letter to your friend. In your letter:
√ say what the learning tool is (give it an interesting name)
√ describe the learning tool and its uses
√ tell the reason why you recommend it
Remember: the learning tool can be a reference book, a website, a program, an app, or whatever you use to help you with your study.
（注意：文中不得出现考生的姓名、校名及其他相关信息，否则不予评分。）
