is the most difficult of all the lessons.I agree. But it’s the most interesting.A.

问题补充：

--- is the most difficult of all the lessons.

--- I agree. But it’s the most interesting.

A. Lesson Three

B. Lesson Third,

C. The Three Lesson

D. Third Lesson

答案：

(答案→)A. Lesson Three

B. Lesson Third,

C. The Three Lesson

D. Third Lesson

解析：句意：第三课是所有课程中最难的。——我也是这么认为。但它是最有趣的。英语中表示事物的编号一般有两种方法，一种是名词+基数词，此时两个单词相当于专有名词，词首字母大写；另一种是the+序数词+名词，此时三个单词相当于短语，首字母不大写。结合语境可知只有选项A结构正确，故选A。