单选题 Lesson Three is (the) most difficult lesson, (but) it isn't (the) most difficult lesson (in) Book Four.
A. the B. but C. the D. in

【正确答案】 A
【答案解析】[解析] 应改为a。a most difficult=a very difficult，而the most difficult则表示最高级的意思。由句子前后意思可知A处不能用最高级。