sked to explain _____ he did to make hisstudents so fascinated with his lessons, the teacher paused and thought deeply.

His luggage includes two large caseswhich ________ as much as 50kg.

______ I had done it I knew I had made a mistake.

Who is the man a blue shirt I have never seen him before.

下列交际用语，正确、得体的一项是（ ）