—John has put forward most challenging question for us to answer.
—Yes, it really is. I have never heard harder one.
A. the; 不填
B. 不填; the
C. the; the
D. a; a
网友回答
A. the; 不填
B. 不填; the
C. the; the
(答案→)D. a; a
解析：第一空后的most不是构成最高级，而是表示“很”、“十分”，故其前用a；第二空也填a，该句为 I have never heard a harder one than this one 的省略。