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

--- 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→)A. Lesson ThreeB. Lesson Third,C. The Three LessonD. 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。

我好好复习下