According to “The Letter A”, the

According to “The Letter A”, the first letter “A” author wrote with his left foot was his road to a new world and his key to________ .
【正确答案】：mental freedom
【题目解析】：原句的意思是“在《The Letter A》这篇课文中，作者用左脚写出的第一个字母是他迈向新世界的第一步，也是他解放思想的关键”。